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:
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.
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.
-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.
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:
DC: Coating possible