Area Forecast Discussion
Issued by NWS Sterling, VA

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000 FXUS61 KLWX 111455 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 955 AM EST Thu Jan 11 2018 .SYNOPSIS... High pressure will continue shifting eastward into the Atlantic today. A broad area of low pressure will approach the area late tonight into Friday, with a cold front expected to pass through the region early Saturday morning. High pressure of Canadian origin builds across the region through early next week. && .NEAR TERM /THROUGH TONIGHT/...
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Warm advection has temperatures rising early this morning. A lower level cloud bank along the Virginia Highlands and up the east side of the Blue Ridge would mark the zone of best upglide. Thus far, radar has been clear, but there is probably some light rain/drizzle occurring on the ridges in our southern CWA (confirmed by media skycam on Elliott`s Knob in Augusta County). Anticipate that rain will gradually develop across the forecast area today, especially along/east of the Blue Ridge where upslope and upglide intersects (likely to categorical PoPs). The Potomac Highlands will be in a shadow, so only chance PoPs there. Latest high resolution guidance suggests lower chances of rain during the daylight hours. Have only taken a step down during the onset period, as upslope light rain/drizzle may not be depicted well. Rainfall rates today shouldn`t be that great. Temperatures will continue their steady rise, even in the absence of sunshine, into the 50s by this afternoon. Incorporating latest guidance only resulted in cosmetic changes to the temperature forecast, but am a little concerned some areas may struggle to rise until we get better flow to the surface. Meanwhile, more efficient mixing could result in temps topping 60. Likewise, will be taking a look to see if we need to taper the rapid rise in temperatures tonight into Friday, especially if we will be seeing rain. The warm advection/isentropic lift increases tonight as a low level jet of 40-50 kt rounds the bend. This will support widespread rain. Precipitable waters increase to nearly 1.5 inches, which is remarkably high for January, so moderate or even locally heavy rain possible. (More on hydro thoughts in the `Hydrology` section below.) There could be enough shear to trigger an isolated rumble of thunder too, but opted to omit due to low confidence of occurrence and limited impact. Low temperatures will occur early in the evening. Then temperatures will keep rising to the upper 50s/near 60 degrees by dawn.
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&& .SHORT TERM /FRIDAY THROUGH SATURDAY NIGHT/...
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The roller-coaster will continue through Saturday. As the low- level jet pulls out Friday morning, there likely will be a break in the rainfall. That brief drying will allow for temperatures to rise well into the 60s, which in turn will permit limited instability to develop (less than 100 J/kg of MUCAPE). A deep latitude trough axis will cross the area Friday afternoon and night, which will power a sharp cold front toward the area. The lift from this system, plus baroclinicity, a negatively tilting trough and right-rear quadrant of the upper jet, will be able to act on the available instability and deep layer shear to support another round of showers. Not sure of lightning will be able to develop, but am most concerned of downward momentum transport at this time, and have included a slight chance of thunder ahead of the cold front. Quick movement will counteract the rich moisture transport, but brief heavy rain remains possible. Temperatures will start to drop after midnight, and will keep dropping through the day Saturday. There is some uncertainty if the moisture will exit before the cold air arrives. That could spell a precipitation-type conundrum, as the cold (subfreezing) air arrives at the surface before it does aloft. At this time, kept it simple...introducing a brief potential of rain/snow. It could be more complex than that. For the western slopes of the Appalachians, there is increasing evidence that the cold air will arrive first, which would support a couple inches of snow to fall Friday night/Saturday morning. Additional (light) snowfall amounts wold be possible during the day Saturday, but it would be scattered and light as the high builds quickly, wind direction is not optimal, and Lake Erie is largely ice-covered. A cold air drains Saturday-Saturday night, there may be a brief diurnal leveling, but otherwise cold advection will dominate. Temperatures aloft will be -10 to -15 C in the 850-925 mb layer. So, lows (not accounting for the wind chill) will be in the teens/lower 20s again by Saturday night.
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&& .LONG TERM /SUNDAY THROUGH WEDNESDAY/... In the wake of the cold front passage on Saturday, upper level troughing will reside across the Eastern U.S. as surface high pressure originating from Canada builds over the region. This high will control our weather elements through Monday with temperatures ranging 10 to 15 degrees below normal and dry conditions expected. Model guidance in good agreement with a clipper system diving southeastward across the Great Lakes on Monday, approaching our region Monday night into Tuesday. From here, guidance differs in the amount of moisture this system is packing as the low tracks to our north. ECMWF has a stronger/deeper upper trough associated with it, and as a result develops a coastal low off the Carolina`s, moving it northward hugging the coast. This solution brings quite a bit more moisture to our area than the GFS solution. Considering it is day 7, will not get in to the weeds at this point, especially with the model to model variations we are seeing, but worth monitoring the next several days. Regardless, snow potential will be there mid week, with at least an upslope snow component along the Allegheny Front, as temperatures remain well below normal. && .AVIATION /15Z THURSDAY THROUGH MONDAY/...
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Flight restrictions will continue to increase through the day as ample moisture and lift overspreads the area, and rain develops. MVFR decks have been transient this morning, but it looks like it will be filling in this afternoon. IFR is possible (would think CHO has highest chance), although it will become more likely tonight. Am a bit uncertain how restrictive fog will be (setup is conducive for its development). Have allowed low cigs to lead the way. In addition, a low-level jet of 40-50 kt will lead to widespread wind shear this afternoon and tonight. There should be a brief break in the showers Friday. Conditions should return to MVFR, but there is a potential that it won`t. Then, a cold front approaches Friday afternoon, and crosses late Friday night. Another round of poor flight conditions in heavy rain (maybe even a rumble of thunder?) and gusty winds will result. Flow will be northwest before sunrise Saturday morning, and will remain so through the day. Ceilings will lift, but gusty winds (20 kt or so) will persist through the day. VFR conditions forecast Sunday and Monday with high pressure building over the terminals. Winds will be a bit gusty out of the northwest early Sunday in the wake of the frontal passage Saturday, but as the high settles closer to our region Sunday night into Monday, winds will turn light and northerly. A clipper system will approach the terminals Monday night into Tuesday, bringing the potential for sub-VFR conditions with snow showers possible.
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&& .MARINE...
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Winds will be light south or southeast this morning, but will increase through the day and evening. Even though cold water will result in poor mixing, the magnitude of wind available just off the deck suggests that Small Craft Advisory criteria will still be reached. Will be issuing an SCA for below Pooles Island, to include most of the larger inlets and the mouth of the Potomac, from midnight to noon. As the low level jet departs Friday, there should be a bit of a lull Friday afternoon (at least below threshold), before winds increase again Friday night ahead of a cold front. SCA likely near the front, and in the favorable mixing Saturday-Saturday night behind it. SCA conditions possible early Sunday in the wake of the cold front passage on Saturday and gusty northwest winds across the waters. As high pressure builds over the area Sunday night into Monday, then residing just offshore on Tuesday, expecting no headlines as winds will be light.
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&& .HYDROLOGY... Warming conditions are expected through the end of the week, along with periods of rain Thursday night through Friday night. Ice break up due to increasing warmth and rain may create localized ice jam issues. Confidence is currently too low and threat too isolated to include in the HWO at the moment, but we will continue to monitor. Around one to one and a half inches of rainfall is anticipated during this period, subject to change based on finer scale details yet to be ironed out. While precipitable water is well above climatology, suggesting that flooding is possible, relatively quick movement and dry antecedent conditions should preclude a widespread hydrologic issue. Thus, certainty not enough for even a Flood Watch at this time. However, that will continue to be assessed. && .CLIMATE... Meteorological winter (which began December 1st) has been very dry so far. Below is a list of the top 5 driest meteorological winters (December 1st through February 28th/29th) on record. Washington DC area (DCA) 1. 2.60 inches (1871-72) 2. 3.32 inches (2001-02) 3. 3.85 inches (1980-81) 4. 4.15 inches (1976-77) 5. 4.76 inches (1873-74) Notes: The winter of 1870-71 had 4.16 inches, but 31 days of data are missing (no data for December 1870). So far, Winter 2017-18 has had 0.66 inches through January 8th. Precipitation records date back to 1871. Baltimore MD area (BWI) 1. 4.03 inches (1976-77) 2. 4.12 inches (1980-81) 3. 4.28 inches (2001-02) 4. 4.30 inches (1871-72) 5. 4.51 inches (1979-80) Notes: The winter of 1870-71 had 2.93 inches, but 31 days of data are missing (no data for December 1870). So far, Winter 2017-18 has had 1.07 inches through January 8th. Precipitation records date back to 1871. Dulles VA area (IAD) 1. 3.24 inches (2001-02) 2. 3.37 inches (1976-77) 3. 4.83 inches (1979-80) 4. 5.18 inches (1980-81) 5. 5.55 inches (2010-11) Notes: The winter of 1962-63 had 0.00 inches, but 38 days of data are missing. The winter of 1961-62 had 5.34 inches, but 34 days of data are missing. So far, Winter 2017-18 has had 0.80 inches through January 8th. Precipitation records date back to 1960. && .LWX WATCHES/WARNINGS/ADVISORIES... DC...None. MD...None. VA...None. WV...None. MARINE...Small Craft Advisory from midnight tonight to noon EST Friday for ANZ531>534-537-540-541-543. && $$ SYNOPSIS...RCM/HTS NEAR TERM...ADS/HTS SHORT TERM...HTS LONG TERM...BKF AVIATION...ADS/BKF/HTS MARINE...ADS/BKF/HTS HYDROLOGY...MM/HTS CLIMATE...DHOF

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