Area Forecast Discussion
Issued by NWS Sterling, VA

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137 FXUS61 KLWX 291408 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 1008 AM EDT Sat Apr 29 2017 .SYNOPSIS... A frontal boundary just north of the Mason-Dixon Line will move south this afternoon and remain over the region through early Sunday...before lifting north as a warm front late Sunday afternoon. A cold front will then cross the mid-Atlantic late Monday. Weak low pressure moves into the region Tuesday before a surface trough develops Wednesday as strong low pressure develops over the Tennessee Valley and heads towards the mid-Atlantic Thursday and Friday. && .NEAR TERM /THROUGH TONIGHT/... Considerable uncertainty remains regarding the forecast for the remainder of the day and tonight. While the airmass boundary (i.e. dew points in the mid 60s) remains north of the area, a couple rounds of decaying convection overnight have sent outflow boundaries across the area, muddling the surface picture. There is enough instability aloft (1000 J/kg MUCAPE on 12Z IAD sounding) that showers and thunderstorms continue to generate. On the whole, models have not handled these processes well, degrading confidence in their output for the remainder of the day. The question remains if the current convection keeps the regeneration trend going, or if the building mid-level ridge and weakening low level jet allow a drier period during the midday to mid afternoon. With the boundaries laid out farther south now, that could provide focus for strong storm development father south later today as well, of course pending midday trends. As mentioned below, ingredients will be in place in the region for strong to severe storms, but triggers (and locations thereof) to battle the ridge and effects of morning storms preclude higher confidence. Previous discussion: There is ample ridging in place today, with plenty of warmth supporting temps to rise to/above 90 degrees. In light of this heat, MUCAPE forecast to increase to 2000-3000 J/kg...or higher...with minimal CIN. But, the northern portion of the forecast area will first need to recover from the early morning rainfall. Have shaved a degree or two off high temps across northern Maryland and eastern West Virginia. In turn, this would provide a differential heating boundary. Believe that would be enough in this volatile air mass for late day showers/thunderstorms to develop. While heavy rain would be primary threat (due to atypical precipitable water for late April), would not rule out localized strong wind gusts or large hail in taller storms (and thunderstorms today likely to be quite tall). SPC concurs, placing most of the forecast area in Marginal risk. Thunderstorms will gradually decrease in coverage upon loss of diurnal heating. However, there will be enough fuel lingering to justify chance PoPs overnight. && .SHORT TERM /SUNDAY THROUGH MONDAY NIGHT/... Similar considerations apply to Sunday`s forecast, ie: the lingering surface boundary and a warm/unstable air mass to its south. An east/onshore flow across northeast Maryland may provide a stabilizing influence, and focus storms in the Potomac Highlands, where terrain/upslope will provide another lifting mechanism. This time, instability will wane with sunset. And, since flow will be onshore, have no reason to dispute slightly cooler high temperatures per SuperBlend...which still are about 10 degrees above normal. On Monday, a deep low pressure system will work from the Plains to the Great Lakes, which will drag a cold front toward the forecast area. GFS/ECMWF timing is a little closer then previous, though differences remain. Focus would be on the afternoon-evening hours. However, instability does not look nearly as strong as during the weekend, but there still will be ample shear and favorable timing for a few strong thunderstorms. && .LONG TERM /TUESDAY THROUGH FRIDAY/... Per the 29/00Z GEFS, the H5 low over the upper Great Lakes early Tuesday lifts northeastward towards Quebec, followed by slight height rises through Thursday. On Thursday, a digging H5 trough moves into the Tennessee Valley, and slowly makes it way towards the northeast U.S. The surface reflection of the upper levels reveal strong northwest flow on Tuesday, followed by a saddle point in the surface pressure field and stalled frontal boundary as low pressure develops in the Tennessee Valley on Thursday and slowly makes its way to the Chesapeake Bay region. It should be noted that the deterministic 20/00Z ECMWF has a closed upper low for the Thu/Fri system as opposed to an open, albeit slow- moving, wave per the 29/00Z GEFS solution. This is to be expected with the GEFS as the various solutions are damping out details of the position of the upper low, and the 29/00Z GFS deterministic has a substantial, deep, cut H5 low. Both the 29/00Z GFS and ECMWF seem to be in harmony with similar solutions. At this time, the sensible outcome of this strong low pressure system looks to be a considerable, slow-moving Nor`easter rainstorm for the region in the Thursday-to-Friday timeframe. && .AVIATION /14Z SATURDAY THROUGH WEDNESDAY/... Outflow from overnight storms have led to development of new storms this morning. Overall coverage is small though and persistence of storms is low, so will have to handle with amendments. High pressure ridging may build enough to allow for a break during the midday hours versus additional scattered storms, but overall confidence in the forecast is low. Ample daytime heat will lead to scattered late day thunderstorms, with local flight restrictions and gusty winds likely. Areal coverage once again the mitigating factor. Threat of flight restrictions will linger overnight/into Sunday morning, with areas of fog possibly developing. Daytime Sunday will carry a renewed thunderstorm risk, although areal coverage and strength of storms potentially could be less. A cold front will cross the terminals Monday afternoon/evening, with yet another opportunity for thunderstorms/local aob IFR. Primarily VFR should prevail Tuesday-Wednesday. && .MARINE... A low level jet was able to partially mix down early this morning across southern portions of the waters and resulted in a few 20 kt gusts. However, this is expected to weaken, and have thus held of on a SCA for now. There will be several opportunities for thunderstorms this weekend, each of which could carry locally strong wind gusts. It will be tough to pinpoint timing with a boundary nearby, but morning activity may wane a bit before the threat renews late today into tonight, and again on Sunday. Finally, a cold front will arrive Monday night. In advance of the front, the gradient wind will increase from the south. That will be the first chance at Small Craft Advisories. Marine Warnings possible prior to that. Northwest flow on Tuesday will likely bring small craft conditions to the waters. As low pressure moves through the region in the Thursday-to-Friday timeframe, additional small craft advisories may be required. && .TIDES/COASTAL FLOODING... Water levels have retreated to near normal, with no complications anticipated this weekend. That should be the case for much of the week. By the end of the week though, a coastal low is likely to cause elevated water levels once again. && .CLIMATE... The time periods to watch will be high temperatures today, and warm overnight lows this weekend. Here are the record highs/warm lows through the weekend: Washington DC... Saturday 29 April...91 (in 1974)/68 (in 1956) Sunday 30 April...92 (in 1942)/67 (in 1983) BWI Airport... Saturday 29 April...91 (in 1974)/67 (in 1956) Sunday 30 April...92 (in 1910)/63 (in 1983) Dulles Airport.. Saturday 29 April...87 (in 1996)/62 (in 1996) Sunday 30 April...86 (in 2007)/64 (in 1983) Also of note...the warmest night time low at DCA in April is 69 degrees. That is being challenged as DCA only dropped to 70 this morning. && .LWX WATCHES/WARNINGS/ADVISORIES... DC...None. MD...None. VA...None. WV...None. MARINE...None. && $$ SYNOPSIS...Lee NEAR TERM...ADS/HTS SHORT TERM...HTS LONG TERM...Lee AVIATION...ADS/Lee/HTS MARINE...ADS/Lee/HTS TIDES/COASTAL FLOODING...HTS CLIMATE...HTS/ADS

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