Area Forecast Discussion
Issued by NWS Sterling, VA

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000 FXUS61 KLWX 181847 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 247 PM EDT Thu May 18 2017 .SYNOPSIS...
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High pressure will remain centered over Bermuda through Friday. A cold front will move south through the area late Friday, then stall to our south on Saturday. The boundary will briefly return north as a warm front Sunday. A stronger cold front passes through from the west on Monday. High pressure will briefly return Tuesday before low pressure and its associated cold front impact the area during the middle portion of next week.
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&& .NEAR TERM /THROUGH FRIDAY/...
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Temperatures are currently trending a couple degrees cooler than yesterday (except in the Baltimore metro) as a result of slightly lower heights/thicknesses and a little deeper moisture. Those same factors are the reason why slightly higher diurnal convection coverage is expected through early evening. However, shear and winds aloft are weak, so showers/storms are likely to be disorganized and not particularly strong. Hi-res models have consistently shown pop-up convection but have also been consistent in placement, keeping it west of the city centers. Tonight will be a muggy, summerlike night. Would not be surprised to see downtown DC and Baltimore stay in the mid 70s. There`s been good model consistency in precip breaking out over southern West Virginia after midnight. I`m not sure how much of that gets far enough east to reach our forecast area, or if any of it can survive east of the divide (similar to what happened this morning further north). Kept low PoPs in the highlands. I`m leaning toward the slower guidance solutions with respect to timing of the cold front; I`ve seen many times in these setups where the slowest solution turns out to be best. That slower solution has the front northwest of us at around 12z Friday, and bisecting the forecast area from Harrisonburg to DC by 18z Friday. With that in mind, I broadened the rain chances out but kept the highest in that Harrisonburg to Fredericksburg corridor. The entire area is now outlooked with a marginal risk in the Day 2 SPC outlook -- marginal definitely being the key word. Shear is still tiny, but south of the boundary, especially if a southwest low-level flow develops (versus westerly), that might be favorable enough to give some robustness to the convection and sustain the cores long enough for marginally-severe wind and hail. Highs tomorrow probably stay a few degrees below today given clouds and continued lowering heights/thicknesses.
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&& .SHORT TERM /FRIDAY NIGHT THROUGH SATURDAY NIGHT/...
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The front settles south Friday evening; again, with slower timing I am keeping higher rain chances in during the evening and even much of the overnight in the Charlottesville area. We will get pseudo-wedged in on Saturday. Not the "perfect" setup but certainly one in which more of a marine layer sneaks in. I lowered highs a couple degrees, and would not be surprised if I am still too warm -- we could stay in the 60s all day. I`ve held on to rain chances in the highlands on Saturday. We`re just going to have to see how far the boundary gets; ensuing shifts may have to add more areas to the Saturday precip possibility and expand it into Saturday night as well.
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&& .LONG TERM /SUNDAY THROUGH WEDNESDAY/...
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High pressure will weaken and move offshore southern New England Sunday and Sunday night, humidity will increase across the region with a southerly flow. There is a slight chance for rain showers, mainly Sunday afternoon into evening. A cold front will move across the region Monday bringing increased chances of thunderstorms that could produce heavy downpours. Dry air returns with a separate area of high pressure building in from the west Monday night through Tuesday. A low pressure system over the lower Great Lakes will intensify as it meanders eastward Tuesday night through Wednesday night. There is a chance for showers and perhaps a thunderstorm or two as pieces of energy rotate around the main storm system. A breezy and dry day expected Thursday with high pressure building in from the west.
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&& .AVIATION /18Z THURSDAY THROUGH TUESDAY/...
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VFR generally expected through the valid TAF period. Can`t rule out an hour or two of MVFR fog at CHO tonight but it was such a marginal possibility that I left it out of the TAF. Could see isolated to scattered SHRA/TSRA at the terminals this afternoon and early evening (least likely at DCA), and then again at all terminals Friday afternoon and evening. Brief restrictions in ceiling and visibility cannot be ruled out. An onshore flow will develop behind the cold front for Saturday into Sunday. With potential for Atlantic moisture to move in from the east, low clouds will likely advect in. How quickly is up for debate, but MVFR ceilings are possible by midday Saturday. Lower ceilings look more likely on Sunday, especially late in the day, as the next system approaches. MVFR or IFR in any heavy showers Sunday night, mainly near MRB and CHO. IFR conditions possible at all terminals Monday. VFR conditions Monday night and Tuesday.
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&& .MARINE...
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Winds have stayed gusty all day on the lower Tidal Potomac and lower Maryland Chesapeake Bay (St. Mary`s County). Ended up just having to keep the Small Craft Advisory up for those waters all day today. The HRRR appears to have nailed increased winds in the waters near Baltimore this afternoon. Using it through early evening necessitated issuing a Small Craft Advisory for the entire Maryland Bay through mid-evening before tapering it back to the lower half of the Maryland Bay overnight. That advisory ends mid-morning on Friday. Scattered showers and storms expected on the waters Friday afternoon and evening; then northerly channeling occurs behind the front, so another Small Craft Advisory was issued for Friday night after midnight. It will likely have to be extended through Saturday morning, but then gusts finally calm down as we get a more stable marine layer moving in. Another cold front moves through on Monday. Could see some southerly channeling ahead of that which might require yet another advisory, and bring scattered showers and storms. Then, no hazards on the waters through midweek.
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&& .CLIMATE...
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Low of 68 at IAD this morning ties the old record set two years ago. Low of 70 at BWI breaks the old (very old!) record. Record highs today not in jeopardy except maybe at IAD. Thursday (05/18) Record Warm Lows: DCA: 72 in 2015 BWI: 68 in 1900 and 1896 IAD: 68 in 2015 Thursday (05/18) Record Highs: DCA: 96 in 1877 BWI: 97 in 1962 IAD: 91 in 1987
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&& .LWX WATCHES/WARNINGS/ADVISORIES...
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DC...None. MD...None. VA...None. WV...None. MARINE...Small Craft Advisory from 1 AM to 6 AM EDT Saturday for ANZ530>534-537-539>541-543. Small Craft Advisory from 8 PM to 10 PM EDT this evening for ANZ542. Small Craft Advisory until 10 AM EDT Friday for ANZ533-534-537- 543. Small Craft Advisory until 10 PM EDT this evening for ANZ530>532.
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&& $$ SYNOPSIS...JE NEAR TERM...JE SHORT TERM...JE LONG TERM...KLW AVIATION...JE/KLW MARINE...JE/KLW CLIMATE...JE

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