Stay Safe!

The Storm Prediction Center is expecting a moderate to high risk for tornadoes in many large metro areas today! Stay safe and take immediate action to protect your life if a tornado warning is issued for your area:

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New Blog

http://ohwxramblings.wordpress.com/

I am trying to change the focus of my blog to shorter but more pattern oriented posts (to increase readability) with more detailed forecasts when conditions warrant. Check it out at the link above! Have a nice day,

Jim.

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Where we Look to go Heading Into December

We look to have a spectacular first half of the holiday weekend across the lower Great Lakes/upper Ohio Valley region, with a strongly positive NAO and positive EPO pumping up a ridge over the central portion of the country. However, the pattern has changed from what dominated the first half of the month (and provided some spectacular stretches of weather) and may continue to change.

For the second half of October into November, an extremely strong Scandinavian ridge was in place over northern Europe. This resulted in wavelengths that caused a downstream –PNA and troughing into western N. American. This forced ridging or at least a zonal pattern over the eastern half of the country, resulting in well above normal temperatures over the region and for a change, a near to below normal month precipitation wise.

However, the current hemispheric 500mb map above shows that the ridge over Scandinavia/northern Europe has flattened significantly. The large negative 500mb height anomalies have shifted from the NE Atlantic to over S. Greenland, and from the western N. Pole to off the northwestern N. American coast. Now, the result for the region is similar, for this weekend at least, more great weather. However, the models are in relatively decent agreement in shifting this Alaskan vortex into a position that allows for some ridging over the western US, and perhaps supports some cooler weather over the lakes/OV.

In the short term, one shortwave will dive down into the Plains Friday into Saturday. The models still differ significantly with the evolution of this feature. The GFS, its ensembles along with the UKMET appear to be northern stream dominant and close off the trough over the Midwest/Great Lakes, supporting some snow showers over the region Sunday into Monday. The Euro, its ensembles and the CMC appear to be southern stream dominant and show a cut off low over the Deep South. Both models gradually move the cut off north, with the Euro not showing much northern stream energy phasing with the cutoff and just a weak low riding up the Appellations with little if any snow. The 0z Canadian shows a much stronger shortwave diving into the upper Midwest Monday, which then phases with the cut off low and results in a relatively deep low riding through the eastern lakes. This results in a swath of accumulating snow over eastern MI and perhaps northern OH, into southern Ontario.

Given the La Nina, I would initially want to give the nod to the GFS/UK and the dominant northern stream. However, with the very strong vortex over Alaska it may be hard to get enough amplification of the northern stream over the next few days to see a cutoff storm over the lakes. This will be very interesting to watch over the next couple days, as the very active Pacific jet stream will give the models fits. A wide array of solutions remains possible.

Moving on, the GFS/ECM ensembles both show the Alaskan vortex moving NW and weakening significantly over the next week:

ECM Ensembles:

GFS ensembles:

Both sets of ensembles appear to show this pattern locking in, meaning that perhaps the pattern is undergoing a shift right now. Both sets of ensembles want to set up some sort of semi-persistent troughing over the Great Lakes/Ohio Valley into early December.

This pattern probably won’t favor a major storm over the region once it locks in (with an outside shot next week as the shift occurs), but it looks like we probably won’t dry torch to start December, and occasional clippers/cold shots may give some shots of light snow, especially downwind of the lakes.

 

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Northeast Ohio Lake Effect Snow Event Two

The models are starting to agree on a solution for Thursday-Friday, and we are now only about 36 hours out from the first flakes of the second lake effect snow event of this winter…

The flow will become northwesterly Wednesday night, and 850mb temperatures will fall to -8 to -10C by 12z Thursday. However, the flow will not be well aligned with forecast skew-t’s showing some directional shear above 850mb, and the atmosphere will be very dry.

Thursday afternoon however, an upper level shortwave will swing through the southern Great Lakes. Mid level moisture will increase some and the flow will become fairly well aligned out of the west ahead of the feature. 850mb temperatures will remain in the -8 to -10C range as the trough approaches and inversions will climb to 10-12k ft per the 12z NAM. This should allow some lake effect/enhanced snow to break out Thursday afternoon. The flow will be generally westerly, so the initial snow should be confined to east of Cleveland, although some scattered/light snow showers may occur elsewhere across northern Ohio as there will be a small surge of mid level synoptic moisture along with decent lift as some good vorticity advects into the region along with decent mid level height falls.

By Thursday evening the shortwave will pass to the east and the flow will become more west-northwesterly. 850mb temps may bottom out between -10 and -12C at this time, for a few hours. There will be some moisture left behind after the shortwave, so some lake effect snows will continue going east of Cleveland into Thursday night.

Ridging will begin to build in late Thursday night. By 12z Friday inversions will likely fall to around 5k feet and the airmass will really begin to dry out, so all that will likely be left is some flurries/light snow showers east of Cleveland. The flow will begin to shift more towards the southwest and temperatures will begin to warm Friday morning, which should effectively end any lake effect over Ohio.

The 12z NAM and ECM are in decent agreement on this scenario. The GFS is more progressive with the trough. Leaned towards the consensus at this time.

As for how much snow may fall, there may be a half inch to an inch of snow east of Cleveland Thursday afternoon as the shortwave passes. There could even be light dustings in the higher elevations of southern Cuyahoga, northern Medina and northern Summit counties as there will be some synoptic moisture and lift with the upper trough. Given the cold 850mb temps/synoptic moisture/lift, it should snow/possibly accumulate down to the lake shore Thursday with the shortwave passage. Thursday evening there will be a brief window for perhaps some decent banding on a WNW flow over the eastern suburbs into the Snow Belt. After midnight lake effect conditions will begin to slowly deteriorate, and I just expect scattered snow showers after mid night out east. If we see a few hours of decent lake effect, I suppose 1-2” more may fall somewhere from eastern Cuyahoga county points east Thursday evening, with relatively light accumulations (half inch or less) after midnight Thursday night. My guess is that the potential max over northeast Ohio is 3” or so, and I do not believe any headlines will be needed.

Will go with this as my guess, for 7am Thursday through noon Friday:

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First Snow of the Year Expected in the Cleveland, OH Area Thursday Night

The models have essentially remained unchanged since yesterday for Thursday night, and some of the short range meso-scale models are now out to Friday morning, adding some clarity to the situation.

The 12z NAM, 12z GFS, and 0z European model runs all show 850mb temps of -5 over the lake and much of Ohio by 0z Friday, and quickly advect in 850mb temps of -6 to -8 over the course of the evening. This is in response to an upper level shortwave and secondary cold front moving through the area Thursday evening. As the shortwave moves through, the models develop a deep cyclonic flow over the lakes and show an increase in mid level humidity, resulting in steep low level lapse rates, high inversions and some mid level moisture to help seed lake effect.

The 12z NAM skew-t for Cleveland right along the lake shore valid 9z Friday, courtesy of wunderground.com shows a relatively uniform WNWrly flow of 20-30 knots in the lowest 20k feet of the atmosphere, with reasonable moisture and instability up through about 13-14k feet. This moisture and instability go through the whole dendrite growth zone. This would certainly support lake effect activity through much of the night.

As the secondary cold front passes through Thursday evening, it is reasonable to assume that scattered showers might break out area wide across a good chunk of northern Ohio as some synoptic lift moves through. After this front passes, the flow will start shifting from westerly to WNWrly and potentially more NWrly by early Friday morning. Given a deep cyclonic flow with deep moisture through the lower-mid levels, high inversions and plenty of instability with lake-850mb delta-t’s of -16 to -18C and the lake-500mb differential around -40C, this supports robust lake effect development over the lake starting around 0z Friday.

Given the wind direction the best banding may initially be concentrated along the eastern lake shore. However, the winds will slowly gain more of a northerly component, shifting the initial band inland into the primary Snow Belt and perhaps favoring another convergence band for a period of time along the western lake shore into the southern/eastern suburbs. If the flow goes almost northwesterly like some models do show by early Friday, the convergence bands may dissipate and be replaced by more of a multi-band setup, which favors the higher terrain inland from the lake.

Accumulations remain tricky…temperatures will start out in the lower 40s Thursday evening which supports rain or mixed showers initially. However, with sufficiently cold temperatures not far above the surface I believe that it will not take long to switch to snow as the evening progresses. Areas along the lake shore and the lower elevations are the exception, where I’m not sure there will be a period of all snow or notable accumulations. Snow should be accumulating in the higher elevations by midnight Friday. My guess is areas above 800 feet will see some accumulations under any heavier bands and that areas above 1000 feet will do well.

The shifting of bands may make it hard for any one area to really cash in. However, the 12z NAM shows .1-.25” of liquid equivalent over most of the Snow Belt and the 12z NMM/ARW mesoscale models show a large area of .25”-.5”+ liquid equivalent. Given instability into the dendrite growth zone and a rather unstable environment I believe over a quarter inch of liquid equivalent could occur where bands persist, and I do think there could be decent snow rates in the higher elevations where the whole column will be at or below freezing overnight. So, I believe a general 2-4” seems possible above 1000’ from eastern Cuyahoga county eastward. Between 800-1000’ 1-3” seems possible in the Snow Belt. Lower elevations will likely remain too warm for much accumulation. The higher elevations in southern Cuyahoga, northern Medina, northern Summit and northern Portage counties could also see 1-3”, especially if winds do become more northwest overnight Thursday into early Friday. The event should wind down Friday morning.

First guess snow map, valid Thursday 5pm-Friday 12pm:

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Updated Weekend Snowstorm Forecast

The subtropical jet stream energy that will supply the moisture for this weekends eastern US snowstorm is producing moderate precipitation over the central Appellations, and the northern stream energy that will combine with this area of precipitation and allow for a strong low pressure to form off of the Mid Atlantic coast is beginning to dig farther south over the Plains. Things will really take off when the strong jet streak over the Dakotas rounds the trough tonight and begins interacting with the southern stream energy.

The 12z global models are in general agreement on a low track from off the lower Mid Atlantic coast to south of Long Island, near the benchmark and then getting a slight nudge east (possibly due to the lack of a strong –NAO block) and likely missing Cape Cod to the east.

The focus of this forecast will be to look closer at where heavy banding may setup and where the rain/snow line will be, and fine tune yesterdays map. The synoptics behind the storm discussed yesterday have not changed.

The storm will begin taking off Saturday morning as the jet streak rounds the bottom of the shortwave over the Ohio Valley, and allows the storm to take on a more neutral to negative tilt. Ahead of the energy will be an area of favorable upper level divergence and positive vorticity advection. Tomorrow morning this area will be over the lower Mid Atlantic, where heavy snows will likely be ongoing over the mountains of VA, WV, western Maryland and beginning to spread into southern PA.

By late afternoon as the storm takes on a negative tilt and intensifies, the upper level left will continue to strengthen. Note how the 500mb height lines (black lines above) spread farther apart over the northern Mid Atlantic and New England, indicated good upper level divergence and lift. In addition, note the very strong mid level vorticity spreading over much of the eastern Mid Atlantic. The area immediately ahead of this strong vorticity will see additional left because of the strong vorticity advecting into the area.

This very strong lift will spread into much of Southern New England during the evening, and in fact overnight the upper level system will close off over New England:

This may aid in the storm slowing down some off the New England coast overnight, maintaining heavier bands of precipitation through much of the night, especially over eastern New England.

In addition to good upper level forcing, as the mid level low tracks up along the immediate coastline, a stark mid level temperature differential will develop along the coast and just inland. This will result in strong mid level frontogenesis, which will contribute to strong low level convergence along the coast and just inland, which will assist in making bands of heavy precipitation possible.

Note how the NAM is depicting very strong upward motion at both the 700mb and 500mb levels (respectively) tomorrow evening over the eastern Mid Atlantic/New England:

This strong upward motion suggests convective bands may be possible, and that heavy precipitation is likely. Both the 12z GFS, NAM, and ECM suggest this strong vertical motion will move well into southern/eastern New England overnight.

In addition, as warm air on a NNE flow off the water collides with colder, denser air moving on a NNW flow over eastern New England a coastal front will form. East of this front a heavy, cold rain is likely. However immediately to the west, the front will aid in focusing heavy snow bands.

Essentially, the lower, mid and upper levels of the atmosphere will support an 8-12+ hour period of moderate to heavy precipitation from the lower Mid Atlantic, north through much of the Mid Atlantic including eastern upstate NY and a good amount of PA into New England. In fact the 12z NAM, GFS, GEFS and ECM all agree on a large swath of 1”+ liquid equivalent over much of the Mid Atlantic and New England.

 

The next question is precipitation type…

As discussed last night, the mid level low track is expected to be from near DC through the Delmarva and southern NJ and then off the Mid Atlantic/New England coasts, expect possibly close to Cape Cod. This means temperatures aloft will generally be cold enough to support snow…HOWEVER, from central NJ points south most of the storm will occur during the day (the sun angle isn’t high, but is enough given the warm ground to keep the boundary layer warm) and it is October, meaning it will take some time to work out the warm air in the boundary layer.

In areas with any elevation (above 750’), snow should stick even during the day fairly well, especially given the heavy, wet nature of the snow. In areas below 500’, I expect mainly rain until late Saturday afternoon, when the setting sun and continued deepening of the storm will allow the lower levels of the atmosphere to try to cool.

I expect DC and Baltimore may end as a period of wet snow, but I’m not expecting accumulations. Philly may see a longer period of snow, but the best snows will be in the higher elevations away from the city. However, over the northern half of NJ and the lower elevations of New England, including Boston and NYC, the situation is not as clear cut…here, I still expect the precip to start as rain…however, more of the storm will occur at night. In addition, as shown above, it appears very strong vertical motion will occur over these areas beginning late Saturday afternoon over NJ and into the evening over New England. This will change rain over to snow, given enough time, even in the low elevations relatively close to the ocean. Thus I believe areas immediately along the coast stay mainly rain/mixed with relatively little accumulation, while areas not very far inland (or high up, for that matter) potentially see a few hours of moderate to heavy snow.

I will reflect this in the forecast, with a sharp gradient of little snow to decent accumulations from the mountains in VA up through DC/Balt and central NJ including Philly, up right through NYC and then extreme southeastern New England.

To the north/west of this line, it appears major accumulations are possible. Given the duration (some areas 12+ hours), moisture involved (waters off the coast still generally 60+, with the Gulf stream still well into the 70s) and very strong dynamics which will ring this moisture out very efficiently that the model forecasts for over 1” liquid equivalent will verify fairly well. This yields a potential foot of snow (even with low ratios) where the precipitation stays snow the whole storm.

Will paint the highest accums over the higher elevations of the southern Mid Atlantic (daytime storm, storm not as dynamic as it will be farther north) and through the higher elevations farther north. However, as the storm rapidly deepens and as night falls will gradually paint in moderate accumulations even in the lower elevations. Will use a blend of the models for the western edge of the snows, as the models at this time are relatively similar with the low track.

Final forecast:

Snowfall by city:

Boston: 1″ (coast)-4″ (western sections)

NYC: 1″ (coast)-4″ (western sections)

Philly: 1-3″

Balt: T-2″

DC: T-1″

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October Winter Storm Looking Increasingly Likely for Mid Atlantic and Southern New England Saturday into Sunday

A slightly (mainly east based) –NAO may help cause significant amplification of a shortwave over the eastern US Saturday. The pieces that will come together and cause cyclonegenesis off the Mid Atlantic coast Saturday are already on the map and over N. America, which increases the synoptic certainty of this forecast.

1. Shortwave moving SW into the Dakotas

2. Small piece of subtropical jet energy currently ejecting out of the southwest.

3. Pseudo 50/50 low pinned by weak ridging trying to stretch into the Davis Strait

4. Small ridge along the US/Canada west coast. NOT a +PNA but enough to help The Dakotas shortwave amplify.

Models are in strong agreement in bringing a shortwave currently moving into the Dakotas southeast over the Plains and then Ohio Valley. The system will be forced south by a polar vortex located near the 50/50 position and by a polar vortex over Canada. The higher heights behind the shortwave will also contribute to it moving southeast.

Note how by tomorrow morning, the shortwave is still relatively harmless looking over the northern Plains. There is very little moisture associated with it and it does not appear to be far enough south to tap any moisture, or cause a storm over the Mid Atlantic.

However, the southern stream energy mentioned above is slowly moving towards the southeastern US. Given a ridge off the southeastern coast, this energy is likely to turn towards the northeast, allowing the northern stream energy to tap it.

Friday night/early Saturday morning, the jet streak will round the bottom of the northern stream energy, allowing the shortwave to take on a more neutral and eventually more negative tilt. The jet will also move over the sub-tropical jet energy, allowing for widespread precipitation to break out over the southeast US and southern Mid Atlantic.

By Saturday morning, the subtropical energy will move off the N. Carolina coast and begin riding northeastward. At this point, it will be in the right-entrance portion of an upper level jet-streak. This, along with the low moving over a sharp baroclinic zone between a continental-polar airmass and tropical airmass over the Gulf Stream will allow for a low pressure system to develop and rapidly deepen off the Mid Atlantic coast Sunday afternoon. The 12z GFS and ECM both deepen the low to sub-980mb by Sunday morning as it pulls away from New England, with the 12z CMC just a few MB weaker.

The upper level jet streak associated with the storm will create a large area of strong upper level lift that will extend inland over the Mid Atlantic and eventually New England Saturday into Saturday night:

This will assist in deepening the low pressure and allowing moderate to heavy precipitation to occur over the coast and areas just inland. At the same time, models are in very strong agreement in significantly falling heights as the storm takes on a negative tilt and significantly deepens. On top of this, the global models show a period of strong PVA moving north through the eastern Mid Atlantic into southern/eastern New England Saturday and Saturday night:

This will also aid in creating enough lift for moderate to heavy precipitation over the eastern Mid Atlantic and southern/eastern New England for a period of time as the low pressure system passes to the east.

In addition to all this, as very strong warm air advection occurs to the east of the deepening cyclone, and cold air advection occurs to the west, a zone of mid level frontogenesis will setup over the same general area:

This promotes lower to mid level convergence and lift.

Factor all of the above together…upper level divergence aided by a jet streak, and low to mid level lift aided by strong positive vorticity advection and mid level frontogenesis, and you get fairly efficient precipitation rates over the eastern Mid Atlantic and eastern/southern New England. In addition, there will be plenty of moisture to ring out given subtropical jet energy in play and a very warm October Gulf stream just off the coast.

The models are showing the storm moving strait out rather quickly, but suggest 12 hours of steady and at times moderate-heavy precipitation are likely over much of the eastern Mid Atlantic and eastern/southern New England. The 12z GFS and ECM agreed with this, with the CMC showing a slightly weaker/east solution and a little less precipitation. The new 18z NAM also agrees with this. All of the above models suggest over an inch of liquid may fall near the coast. Models are disagreeing with how far west precipitation may get, but given that mid-level frontogenesis and decent height falls will occurs back west into central PA/Upstate NY and into central VT, NH and Maine, as shown above, have to believe measurable precipitation will occur at least that far north/west.

The next big question is precipitation type…

The 12z ECM tracks the mid level low just south of DC, over the Delmarva and southern NJ, and then keeps it south/east of the rest of the Mid Atlantic. The 12z GFS was similar, if not a tad farther south during the day Saturday. This in general means temperatures aloft will be cold enough for snow to the coast including all of the big cities…however, for much of the Mid Atlantic, the bulk of the precipitation will fall during the day, during October.

GFS on top, ECM on bottom. 12z runs valid 18z Saturday.

Note how both the GFS and ECM show temperatures in the big cities generally in the mid 30s to near 40, with temperatures well inland generally in the low to mid 30s. This leads me to believe areas near the coast and at lower elevations will be too warm for all snow during the daylight hours Saturday, and likely won’t see accumulations until late afternoon or early evening as the sun goes down. This also leads me to believe that higher elevations will see the highest accumulations, and that New England may do better than the Mid Atlantic due to more of the storm occurring at night over New England.

Essentially, this is what I did with accumulations:

Appellations in the southern Mid Atlantic into Northern Virginia:

-3-6” of snow. Bulk of the QPF will fall to the north/east of here, however conditions will be cold enough for accumulating snow given time of day and elevations.

Lower elevations/coastal Plain of southern Mid Atlantic:

-Expect a coating of wet snow. Time of day precipitation will fall and elevation will not favor accumulating snow. Dynamical cooling may cool the column enough for some snow during the heaviest precipitation. A changeover to and possible light coating of snow may occur in the late afternoon/early evening as the sun goes down, however the moisture will begin pulling out.

Inland southern Mid Atlantic:

-Light accumulations. Precip type will flip faster than coast, but same principles apply.

Northern Mid Atlantic (lower elevations):

-Light accumulations, potentially to the coast. Precip type through mid to late afternoon will be mainly rain, but a flip to snow will probably occur by late afternoon/early evening. A couple-few hours of moderate snow may occur. The ground will be warm, but by evening the snow may stick if it comes down hard enough.

Inland/elevated portions of the northern Mid Atlantic:

-Moderate accumulations, highest above 1000’. It will not take much elevation to see accumulating snow during the day given the cold air aloft and strong dynamics at work. If it snows hard enough it should stick, especially at the higher elevations and by late afternoon in the lower elevations. Snow will last a few hours into the evening which will help accumulation.

New England:

-Coastal Plain may change to snow in the evening, but will show light accumulations due to a warm boundary layer here.

-Inland: Most of precipitation will occur from late Saturday afternoon into the evening, which favors accumulating snow for much of the storm. Local amounts of over 6” certainly possible.

-Higher elevations (mainly southern New Englnad): Heavy accumulations possible given higher elevations (higher ratios), time of day and dynamics at play.

What could go wrong?

-I doubt the storm ends up being significantly flatter/weaker than currently shown, especially as the models if anything are trending stronger/more phased.

-A stronger/more phased solution may bring slightly warmer air farther west. This would significantly limit any snow near the coast and at the lower elevations.

Snow map:

This map, as always is subject to change. Small changes in storm track, intensity, temperatures and/or precipitation rates/amounts can have fairly large impacts on such a sensitive situation, an accumulating October snow. Areas above 1500’ may see more widespread 6”+ amounts than what’s shown, especially in the northern Mid Atlantic/New England.

Elevation was depicted as well as possible given the large size of the map and software used to create it. However, if in doubt, the higher you go, the more snow you should expect.

Snowfall by city:

Boston: 1-3”

NYC: 1-2”

Philly: 1-2”

Baltimore: Coating

DC: Coating possible

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