541  
FXUS61 KLWX 170127  
AFDLWX  
 
AREA FORECAST DISCUSSION  
NATIONAL WEATHER SERVICE BALTIMORE MD/WASHINGTON DC  
927 PM EDT WED AUG 16 2017  
   
SYNOPSIS
 
 
HIGH PRESSURE OVER THE REGION WILL SLOWLY MOVE EAST OFF THE  
COAST TONIGHT. A WARM FRONT WILL MOVE NORTHWARD THURSDAY INTO  
THURSDAY NIGHT FOLLOWED BY A COLD FRONT THAT SHOULD PASS THROUGH  
THE REGION FRIDAY NIGHT. WEAK HIGH PRESSURE WILL BUILD INTO THE  
AREA OVER THE WEEKEND.  
 

 
   
NEAR TERM /THROUGH THURSDAY/
 
 
WEAK STALLED FRONT REMAINS STRETCHED WEST TO EAST ACROSS THE  
MIDDLE OF THE FORECAST AREA. ISOLATED SHOWERS HAVE BEEN FORMING  
IN THIS ZONE, ALTHOUGH A GENERAL DISSIPATING TREND IS EXPECTED  
TO CONTINUE AS NOCTURNAL STABILITY INCREASES. SHARP INVERSION  
AROUND 14KFT (BELOW THE FREEZING LEVEL) ON THE 00Z IAD SOUNDING  
EXPLAINS WHY NO LIGHTNING HAS BEEN OBSERVED.  
 
OVERNIGHT, WARM ADVECTION WILL DEVELOP ALOFT, RESULTING IN AN  
INCREASE IN MID-LEVEL CLOUDS, BUT PERHAPS NOT UNTIL LATE. THUS  
QUESTIONS REMAIN ABOUT LOW CLOUDS AND FOG. THE NAM HAS  
DEFINITELY BE THE MOST PESSIMISTIC MODEL IN TERMS OF BOTH, WHILE  
THE GFS ONLY SUGGESTS SOME CLOUDS BETWEEN 5-7KFT. CONSIDERED  
SREF AND NARRE PROBS (WHICH ARE LOW FOR SIGNIFICANTLY REDUCED  
VISIBILITY) AND THUS KEPT THE FLAVOR OF THE FORECAST SIMILAR,  
WITH PATCHY FOG IN THE TYPICAL PRONE LOCATIONS.  
 
DEW POINTS SUGGEST LOWS WILL BOTTOM OUT IN THE MID 60S TO LOW  
70S. MAY NEED TO ADJUST UP A LITTLE, ESPECIALLY IN AREAS OF  
THICKER CLOUDS AND TO THE SOUTH OF THE STALLED BOUNDARY.  
 
BOUNDARY WILL SLOWLY MOVE NORTHWARD BACK ACROSS THE AREA DURING  
THE DAY THURSDAY. PW'S WILL SURGE SIGNIFICANTLY, WITH VALUES  
LIKELY TO BE ABOVE 2 INCHES BY LATE AFTERNOON. A WEAK SHORTWAVE  
AND VORT MAX WILL ALSO CROSS THE REGION. HOWEVER, CLOUD COVER  
WILL BE ABUNDANT, LIMITING CAPE AVAILABILITY SOMEWHAT. THE  
RESULT IN SOUNDINGS IS A SKINNY CAPE PROFILE, MORE SUGGESTIVE OF  
A HEAVY RAIN/FLASH FLOODING THREAT THAN A SEVERE THREAT, BUT  
THE LATTER CAN'T BE COMPLETELY RULED OUT GIVEN AROUND 25 KNOTS  
OF SHEAR. THE BEST OVERLAP OF SHEAR AND INSTABILITY WILL BE IN  
THE NORTHWESTERN QUADRANT OF THE AREA...WITH THE HIGHEST CHANCES  
OF STORMS IN GENERAL NEAR AND WEST OF THE BLUE RIDGE. WITH THE  
CLOUDS, TEMPS SHOULD END UP A LITTLE COOLER THAN WEDNESDAY, BUT  
STILL A VERY WARM AND STICKY DAY.  
 

 
   
SHORT TERM /THURSDAY NIGHT THROUGH FRIDAY NIGHT/
 
 
AFTER THE TROUGH/MAX PASSES AND INSOLATION WANES, EXPECT  
PRECIPITATION TO WIND DOWN DURING THE EVENING, BUT WITH A VERY  
MOIST ENVIRONMENT, A FEW SHOWERS MAY LINGER. WITH THE CLOUDS AND  
HIGH HUMIDITY, LOWS WILL STAY VERY HIGH, WITH 70S BEING  
WIDESPREAD. ANY AREAS THAT SEE A LITTLE CLEARING MIGHT SEE SOME  
PATCHY FOG. DEPENDING ON THE LOCATION OF FORCING/BOUNDARIES AND  
AVAILABLE INSTABILITY ALOFT, THERE COULD BE A CONTINUATION OF  
THUNDERSTORMS WITH LOCALLY HEAVY RAIN LATE IN THE NIGHT INTO  
FRIDAY MORNING DUE TO AN APPROACHING LOW LEVEL JET. CURRENT  
MODEL PROJECTIONS SUGGEST THIS WOULD BE OVER THE NORTHEASTERN  
QUADRANT OF THE AREA.  
 
FRIDAY THE NEXT COLD FRONT APPROACHES. WITH MORE OPPORTUNITY FOR  
SUN EARLY IN THE DAY AND COOLING ALOFT, CAPE SHOULD BE MORE  
PLENTIFUL, WITH SHEAR INCREASING TO AROUND 30 KNOTS. THIS SHOULD  
RESULT IN MORE OF A SEVERE THREAT VERSUS FLASH FLOOD THREAT,  
THOUGH WITH PW'S STILL OVER AN INCH AND A HALF, ANY CELLS THAT  
TRAIN COULD STILL POSE A FLOOD RISK. WITH MORE SUN, HIGHS  
SHOULD APPROACH IF NOT REACH 90 IN THE WARMER SPOTS.  
 
FRONT PASSES ACROSS THE REGION AT NIGHT, WITH CHANCE OF PRECIP  
RAPIDLY DECLINING. HOWEVER, WARMTH AND MOISTURE AT LOW LEVELS  
WILL BE SLOWER TO EXIT, WITH LOWS LIKELY IN THE 70S NEAR I-95,  
COOLER IN THE MOUNTAINS.  
 

 
   
LONG TERM /SATURDAY THROUGH WEDNESDAY/
 
 
A COLD FRONT WILL BE STALLING TO OUR SOUTHEAST AS A WEAK HIGH  
PRESSURE BUILDS OVER OUR AREA ON SATURDAY. UPPER LEVEL ENERGY  
COULD BRING SOME SHOWERS SATURDAY NIGHT, WITH DRYNESS RETURNING  
ON SUNDAY. HIGH PRESSURE MOVES OFFSHORE AS THE FRONT SLOWLY  
PUSHES AWAY FROM US AND STALL OVER THE CAROLINAS SUNDAY INTO  
MONDAY. SOUTHERLY FLOW WILL ADVECT HUMIDITY INTO OUR AREA WITH  
INCREASING TEMPERATURES INTO MONDAY. A PRESSURE TROUGH COULD  
ENHANCE AFTERNOON SHOWERS AND THUNDERSTORMS MONDAY INTO TUESDAY.  
A COLD FRONT WILL APPROACH FROM THE WEST ON WEDNESDAY WITH  
ADDITIONAL SHOWERS AND THUNDERSTORMS POSSIBLE. HIGH TEMPERATURES  
WILL BE IN THE MID TO UPPER 80S... SOME 90S AND 70S AT HIGHER  
ELEVATIONS.  
 

 
   
AVIATION /01Z THURSDAY THROUGH MONDAY/
 
 
IT LOOKS LIKE ANY REMAINING SHOWERS NEAR A STALLED FRONT WILL  
AVOID THE TERMINALS. THERE CERTAINLY ARE A RANGE OF SOLUTIONS  
REGARDING CIGS/VSBY OVERNIGHT. NAM HAS BEEN THE MOST PESSIMISTIC  
WITH MOISTURE OVERRUNNING WITH WIDESPREAD IFR OR LOWER, WHILE  
THE GFS ONLY SUGGEST CLOUDS BETWEEN 5-7KFT. GIVEN LOW IFR PROBS  
IN THE NARRE AND SREF, HAVE KEPT FLAVOR OF PREVIOUS FORECAST  
WITH MVFR AT IAD, MRB, AND CHO. ONE CAVEAT TO THE FORECAST: IF  
LOW CLOUDS DO DEVELOP, THEY MAY TAKE SOME TIME TO LIFT THURSDAY  
MORNING, UNTIL A WARM FRONT CAN LIFT TO THE NORTH.  
 
MAIN CONCERN THURSDAY AND FRIDAY WILL BE THUNDERSTORMS, MOSTLY  
DURING THE AFTERNOON AND EARLY EVENING HOURS, WHICH COULD RESULT  
IN BRIEF INTERVALS OF IFR CIGS AND VIS, ALONG WITH GUSTY WINDS.  
 
VFR CONDITIONS EXPECTED SATURDAY INTO MONDAY, WITH SHORT  
PERIODS OF SUB-VFR CONDITIONS SATURDAY NIGHT AND MAYBE MONDAY  
AFTERNOON DUE TO POSSIBLE SHOWERS AND THUNDERSTORMS.  
 

 
   
MARINE
 
 
SUB-SCA AT PRESENT, WITH VARIABLE WIND DIRECTIONS DUE TO A  
STALLED FRONT AS WELL AS ISOLATED SHOWERS WHICH HAVE PRODUCED  
OUTFLOW BOUNDARIES. SOUTHERLY WINDS SHOULD DEVELOP THURSDAY  
MORNING. AN APPROACHING COLD FRONT LATER THURSDAY INTO FRIDAY  
LOOKS LIKELY TO BRING SOUTHERLY CHANNELING TO THE BAY AND LOWER  
POTOMAC. SCA RAISED STARTING LATE THURSDAY AND LIKELY WILL NEED  
TO BE EXTENDED THROUGH FRIDAY. OTHER MARINE CONCERN WILL BE  
THUNDERSTORMS EACH OF THE NEXT TWO DAYS, WITH GUSTY WINDS A  
CONCERN.  
 
MAINLY DRY CONDITIONS EXPECTED SATURDAY INTO MONDAY, WITH  
PERIODS OF SHOWERS AND THUNDERSTORMS POSSIBLY DEVELOPING  
SATURDAY NIGHT AND MONDAY AFTERNOON. WIND GUSTS ARE EXPECTED TO  
STAY BELOW THE SMALL CRAFT ADVISORY THRESHOLD.  
 

 
   
TIDES/COASTAL FLOODING
 
 
ANOMALIES ARE AROUND A HALF FOOT TONIGHT, WHICH WILL KEEP WATER  
LEVELS BELOW FLOOD STAGE. A RETURN OF SOUTHERLY WINDS WILL  
BRING RISING ANOMALIES BACK TO THE REGION ON THURSDAY, WITH  
POTENTIAL MINOR FLOODING AT SENSITIVE SITES BY THURSDAY  
AFTERNOON/EVENING AND CONTINUING INTO FRIDAY.  
 

 
   
LWX WATCHES/WARNINGS/ADVISORIES
 
 
DC...NONE.  
MD...NONE.  
VA...NONE.  
WV...NONE.  
MARINE...SMALL CRAFT ADVISORY FROM 4 PM THURSDAY TO 6 AM EDT FRIDAY FOR  
ANZ533-534-537-541-543.  
 

 
 

 
SYNOPSIS...RCM  
NEAR TERM...ADS/RCM  
SHORT TERM...RCM  
LONG TERM...IMR  
AVIATION...ADS/IMR/RCM  
MARINE...ADS/IMR/RCM  
TIDES/COASTAL FLOODING...ADS/RCM  
 
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