Area Forecast Discussion
Issued by NWS Sterling, VA

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000 FXUS61 KLWX 190132 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 932 PM EDT Fri May 18 2018 .SYNOPSIS... A stationary front over southern Virginia will slowly lift northward through Saturday. A cold front approaching from the Ohio Valley will cross the region from west to east Sunday, then stall to the south early next week. && .NEAR TERM /UNTIL 6 AM SATURDAY MORNING/...
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A steadier band of rain has developed tonight over central Virginia. The 850 mb warm front has lifted to about the Potomac River, but the surface front is still south of Richmond. This is creating a rather elongated region of upglide, with PWATs of 2+ inches advecting into the cool sector. Expect the steady rain with embedded heavier elements to continue to expand northwest overnight, with the heaviest rain over central Virginia. Saturated ground and waterlogged trees are causing trees to topple across much of the area this evening, even in the absence of significant wind. This will be a hazard to those traveling tonight, or to those who live in homes surrounded by trees. Additional rainfall overnight should range from 1 to 2 inches (highest near and east of the Blue Ridge), with locally higher amounts possible. The radar beam is overshooting a decent amount of the rain, with radar estimates of around 0.6 inches in the last 24 hours not quite half of what was measured by observers on the ground. A Flood Watch remains in effect until 8 AM Saturday morning, but some areas near and east of I-95 could still see some heavier downpours through midday, so that may need to be extended.
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&& .SHORT TERM /6 AM SATURDAY MORNING THROUGH SUNDAY NIGHT/... Early morning showers will shift east of the area during the afternoon. Warm front will lift north into PA in the afternoon with a few breaks in the clouds possible in the west allowing for weak destabilization and perhaps a couple of t-storms. The front will drop south again on Sunday keeping the risk of showers. && .LONG TERM /MONDAY THROUGH FRIDAY/... A chance of showers and afternoon/evening thunderstorms will continue into Monday and Tuesday, as a cold front moves just to our south and stalls out. Both GEFS and EPS ensembles show a potential for 0.25-0.50 inches of rainfall on throughout this period, primarily in the evening. The highest amounts will generally be along the eastern slopes of the Blue Ridge. Temperatures on both days will be in the upper 70`s to low 80`s. The forecast for Wednesday into Thursday is very uncertain at this point. What is known is that a stronger low pressure system will pass well to our north, dragging a cold front through our area in its wake. The timing of this front is highly variable at this time, so will keep chance PoPs in the forecast for Wednesday, moving to slight chance for Thursday. The key thing to note here is that, upon passage, this front should bring in some much drier air behind it. Temperature forecasts for this scenario will almost certainly change, but for now, highs should be in the low 80`s on Wednesday, followed by a slight cool down on Thursday in the wake of the aforementioned cold front, with temps in the upper 70`s. && .AVIATION /02Z SATURDAY THROUGH WEDNESDAY/... IFR/LIFR conditions tonight in rain/showers, then improving to MVFR Sat afternoon. VFR conditions expected during the first half of next week. Mainly VFR conditions forecast Monday and Tuesday, with the only threat of sub-VFR conditions being a passing shower over the terminals. Winds will be light and favor a southerly trajectory. A frontal boundary will drop southward through the terminals late Monday, before stalling south of the terminals through mid week. A moist easterly flow north of the aforementioned boundary will bring the potential for sub VFR VIS/CIGs through mid week. && .MARINE...
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SCA through Sunday for all waters on blustery southeast flow. Generally sub-SCA conditions expected Monday through Tuesday, with winds remaining light. A frontal boundary will cross the waters late Monday which will help increase shower activity, resulting in the potential for periodic gusty winds.
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&& .HYDROLOGY...
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Steadier rain continues to pivot northward across central Virginia. Radar is underestimating amounts (especially at far ranges, likely due to overshooting of shallower heavy rain elements; KLWX Z/ZDR is now in tropical to try and account for this). Rain has a little more organization than in previous days due to better frontogenetical forcing and moist advection around the 850 mb level. Easterly flow has also strengthened which has increased humidity levels, so even with a lack of deeper convection rainfall rates could easily exceed half an inch to near one inch per hour (especially near/east of I-95). May have to consider extending the Flood Watch east of the Blue Ridge depending on how persistent the steadier rain is through Saturday morning. Of note, St. Mary`s River near Great Mills reached its highest stage since Sandy in 2012, and more rain is expected overnight, causing river levels to rise further.
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&& .TIDES/COASTAL FLOODING...
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Water levels are running around a foot to 1.5 ft above astronomical predictions on the Bay and lower tidal Potomac. On the upper tidal Potomac, however, those departures increase to 2 ft or more. For the upcoming tide cycle, that should equate to minor flooding on both sides of the river. Coastal Flood Advisories remain in place. By the day on Saturday, it gets more complicated as there will be freshwater input as well. This will affect Georgetown more than southwest DC or Alexandria. -For Alexandria: have extended the Advisory for another two tide cycles (late Saturday night). It could be necessary for a third cycle, but there is lesser confidence in the duration. -For Washington DC: Georgetown will be the driver due to the freshwater contribution, where moderate flooding is forecast for three cycles (with levels not dropping below minor). At the southwest waterfront, best confidence for moderate flooding will be during the day Saturday; subsequent cycles may only be minor flooding. Similar to Alexandria, have duration uncertainties. Therefore, have issued a Warning for the Saturday day and night tide cycles, and left the Watch in place for Sunday. Elsewhere on the Bay, minor flooding will be a concern this weekend during the times of high tides. Will be assessing upcoming guidance and could be making Advisory decisions overnight.
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&& .LWX WATCHES/WARNINGS/ADVISORIES...
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DC...Flood Watch until 8 AM EDT Saturday for DCZ001. Coastal Flood Warning from 6 AM Saturday to 8 AM EDT Sunday for DCZ001. Coastal Flood Watch from Sunday morning through Sunday afternoon for DCZ001. Coastal Flood Advisory until 6 AM EDT Saturday for DCZ001. MD...Flood Watch until 8 AM EDT Saturday for MDZ003>006-011-013-014- 016>018-501>508. VA...Flood Watch until 8 AM EDT Saturday for VAZ025>031-036>040- 050>057-501>508. Coastal Flood Advisory until 8 AM EDT Sunday for VAZ054. WV...Flood Watch until 8 AM EDT Saturday for WVZ050>053-055-501>506. MARINE...Small Craft Advisory until 6 PM EDT Sunday for ANZ530>543.
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&& $$ SYNOPSIS...DHOF NEAR TERM...LFR/DHOF SHORT TERM...LFR LONG TERM...CJL AVIATION...LFR/DHOF/CJL MARINE...LFR/DHOF/CJL HYDROLOGY...DHOF TIDES/COASTAL FLOODING...HTS

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