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January 12-14, 2019 Storm Discussion STORM MODE

WxUSAF

By WxUSAF,
in Mid Atlantic

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WxUSAF

WxUSAF

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Posted
9 minutes ago, Bob Chill said:

3k looks mostly like the euro. Especially in md/dc

 

3 minutes ago, BTRWx's Thanks Giving said:

D.C. jackpot

I like how both are showing that max QPF band oriented E-W near dc. Especially since I fully expect it  to shift 30 miles north to MBY.

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Bob Chill

Bob Chill

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Just now, EastCoast NPZ said:

What do we want to see on that skewT wrt the DGZ?  How do you determine if good or bad?

Look on the left hand column. This is a great sounding for high ratio snow. When the longest bars are in between the DGZ delineations it means great dendrite production. 

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Sparky

Sparky

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1 hour ago, MillvilleWx said:

That was a really nice Euro run to round out the 12z suite. I am interested to see if everything holds heading into tonight with the 0z guidance, but I'll have to take a look at that tomorrow morning since I have to wake up at 315 CST for work. So, for what we've all seen at our disposal, here's my first call forecast below with a short write up.

Write Up

So far this morning we've seen a positive short term trend with the handling of the shortwave progression through the South-Central US. The system in question was a bit more robust than previous thinking for today as we saw more rain and convection blossom across West Texas than model guidance was indicating into last night. This leads to believe the southern stream energy will be a bit better organized as it ejects eastward along with a strong southern jet influence downstream. Models have increased QPF in successive runs through the Mid Mississippi Valley and Tennessee/Ohio River Valley's in conjunction with the positive departures in the southern stream moisture field. Expect the radar to continue to expand as the current shortwave remains consolidated and motions to the east with greater upper divergence on the northern side of the energy and stronger PVA swinging through the aforementioned areas. 

As we enter into tomorrow, dew point depressions across the Mid Atlantic will range between 10-15 degrees by 12z given the latest model consensus. By the morning, H85 wind field begins to shift into a southerly component and weak moisture advection on the leading edge of the disturbance will initiate a slow increase in regional dew points and lowering the dew point depression back toward 8-12 degrees by 18z tomorrow. By this time, we'll begin to see the initial QPF field entering the western and southwestern VA and WV areas with precipitation shield developing on the lee side of the Apps and adjacent Allegheny front. H7 moisture advection will rise by this time and we'll begin to see radar returns poking into the Shenandoah and rapidly expanding to the northeast. The low and mid layer moisture layers will become saturated over time and flurries should begin from southwest to northeast into a greater part of the sub forum with light snow field expanding through the valley's. By late tomorrow afternoon and evening, we'll see more light and moderate returns across VA and west-central MD with accumulation beginning immediately given the cold antecedent air mass coupled with sub-freezing ground temperatures. This is where the accumulation period will begin for basically everyone except maybe the eastern side of the Bay where light snow will spread over by nightfall just given the timing. 

By Saturday night through Sunday morning, WAA regime will be at its peak with modest jet coupling centered over the Central Mid Atlantic and initial PVA from the shortwave moving over or just south of the region. The coupled jet dynamics and divergence generated by the shortwave will create a period of mesoscale banding structures over the area with moderate snowfall likely within areas under the developing bands. There may be some heavier bands that develop over the Shenandoah and adjacent BR due to the closer proximity to the strengthening jet max over the Tennessee Valley with placement within the left exit region of the jet. Coupled with orographic lift in the general area, snowfall could become heavy and max out with higher ratios locally given the expected lift and higher omega levels within the DGZ. Ratios will likely settle around 10-11:1 on average for everywhere south of I-70 for the WAA regime, BUT within the overall setup, expect some higher ratios at times due to the mesoscale banding structures created by the mid and low level convergence centered by the south to southeasterly low level wind field and the compression of the colder air to the north pressing with the feed from the arctic high to the north. This is why I am not ruling out a secondary max over the favored elevated areas near the PA line and local Parr's Ridge area due to the orographic element in play. Average ratio north of I-70 will run 12-15:1 through the event and will see the benefit of the local effects and the best convergence area due to the confluence position just to the north. 

By Sunday morning and beyond, we will have to keep our eyes on the developing coastal along the Northern reaches of OBX up to ORF as that will be what could elevate totals for some as we add a deformation type setup to the mix and enhancement QPF maximum within the northwest quadrant of the low's precip field. As of now, current baroclinic ribbon has adjusted north slightly from the past 24 hours of runs and has been hinted by some guidance, especially the newest Euro run where the NW quadrant QPF field expanded to include just east of DC proper and across the Eastern shore of MD and Southern DE and northern Tidewater of VA. These areas will have to keep an eye on the latest model trends as this may evolve into a more impactful storm with elevated stripe of snowfall totals within the best banding in the deformation shield. It is still possible to get the I-95 corridor within the western fringes of the deformation axis, which would in short play a significant role in the accumulation forecast going forward. 

Things we want to see going forward next 24 hours is model agreement with the placement of the low center as it evolves on the coast near ORF and the continued dual jet structure with strengthening jet trailing the H5 trough. Euro was very close to a viable solution to get a significant portion of the sub-forum in WSW criteria snowfall. Canadian has been gung-ho for quite a while, so I'm curious to see if there will be any adjustments in the precip field (drying out or staying elevated with QPF field), as well as the placement of the deformation snows on the northwest side of the surface low developing off the coast. HRDPS, NAM Nest, RAP, and if you're daring HRRR will be viable products to use starting tomorrow morning to analyze trends in synoptic scale features and more important, any mesoscale features that will undoubtedly play a significant role in total snowfall across the region.

Snowfall Forecast

So, with all that in mind, here are my current feelings in terms of snowfall. I will include a map with zones to outline my thoughts and boom/bust potential given the latest data.

1568261365_SnowfallZoneForecast1_11-13_19.thumb.PNG.d1505b9c7bce7f7a402094f873852a6e.PNG

 

Zone 1:  Havre de Grace: 1-3"

              Bel Air: 2-4"

              Baltimore County Hereford Zone: 3-5"

              Timonium/Towson/Western Baltimore County: 3-6"

              Eastern Baltimore County: 2-5"

              Baltimore City: 2-4"

              Northern Carroll: 3-7" with local highest along Parr's Ridge (Manchester/Westminster/Hampstead/Mt Airy)

              Southern Carroll: 3-6"

              Howard County: 2-6" Local maximum in Western HoCo

              Eastern Frederick County: 3-6"

              Northwest MoCo: 3-6"

 

Zone 2:  East of Bay including all MD/DE counties: 3-8" with highest totals across interior areas including Easton/Kent Co/Interior Sussex Co. 2-5" along the Atlantic shore

             AA County: 3-7" with max to the south

             South-Central MD counties, including PG Co: 3-7"

             Central and Southern MoCo: 3-6"

             NW DC: 3-6"

             SE DC: 3-7"

             Center City DC: 3-5"

             NoVa: 4-7"

             Frederick County: 3-8" with highest in Western Fdk Co and along South Mountain/Catoctins

             BWI: 3-6"

             IAD: 3-7"

             DCA: 2-5"

 

Zone 3:  Valley Areas with elevation below 1200': 4-8"

              Blue Ridge/Allegheny/Apps with elevation above 1200-2400': 5-10"

              Blue Ridge/Allegheny/Apps with elevation above 2400': 6-12"

              

Zone 4:   Shenandoah Valley areas: 3-7"

               Central Va: 3-8" with highest to the east

               Southern MD: 4-8"

               SBY to OC: 2-6"

               Richmond and Areas south of Dashed Line: 1-5" with higher to northwest of RIC. Mixing issues possible

 

 

 

Love your Parr's Ridge mesoscale analysis!!

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