657  
FXUS61 KLWX 241439 AAA  
AFDLWX  
 
AREA FORECAST DISCUSSION...UPDATED  
NATIONAL WEATHER SERVICE BALTIMORE MD/WASHINGTON DC  
1039 AM EDT TUE APR 24 2018  
   
SYNOPSIS
 
 
LOW PRESSURE TO OUR SOUTH WILL IMPACT THE WEATHER TODAY. THE LOW  
WILL SLOWLY MOVE UP THE MID-ATLANTIC COAST TONIGHT BEFORE  
PASSING BY TO OUR EAST WEDNESDAY. A COLD FRONT WILL PASS THROUGH  
WEDNESDAY EVENING AND HIGH PRESSURE WILL RETURN FOR THURSDAY.  
LOW PRESSURE MAY IMPACT THE AREA OVERNIGHT THURSDAY INTO FRIDAY  
BEFORE A COLD FRONT PASSES THROUGH SATURDAY. HIGH PRESSURE WILL  
RETURN BY THE END OF THE WEEKEND.  
 

 
   
NEAR TERM /THROUGH TONIGHT/
 
 
CUTOFF UPPER-LEVEL LOW PRESSURE OVER TN THIS MORNING WILL CATCH  
UP TO THE SFC LOW TONIGHT WITH UPPER SYSTEM WEAKENING AND  
PHASING WITH NORTHERN STREAM TROUGH DROPPING FROM THE EASTERN  
GREAT LKS.  
 
STEADY RAIN EXPECTED THIS AFTERNOON THROUGH MIDNIGHT DUE TO  
DECENT ONSHORE FLOW AND WARM AIR ALOFT OVERRUNNING COOLER AT THE  
SFC. STILL EXPECTING A A GOOD SOAKING RAIN WITH TOTALS BETWEEN  
1-1.5 INCHES SOUTH OF I-66 AND 2+ ON THE EASTERN SLOPES OF THE  
BLUE RIDGE WITH LESSER AMOUNTS OVER NORTHEAST MD.  
 
PREVIOUS AFD...  
 
A DEEP MOISTURE FETCH FROM THE GULF STREAM IS ADVECTING INTO  
THIS SYSTEM. THE WARM AND MOIST AIR FROM THE GULF STREAM WILL  
OVERRUN SURFACE COOLER AIR RESULTING IN A SOAKING RAIN FOR THE  
ENTIRE AREA. THE TRICKY PART WILL BE THE TIMING OF THE RAINFALL  
ARRIVAL. RAIN IS EXPECTED TO FILL IN EARLY THIS MORNING ACROSS  
THE POTOMAC HIGHLANDS INTO CENTRAL VIRGINIA. A SOAKING RAIN IS  
EXPECTED FOR THE MORNING RUSH ACROSS THESE AREAS. THIS BAND OF  
RAIN WILL GRADUALLY SPREAD NORTH AND EAST...MOST LIKELY MAKING  
IT INTO THE WASHINGTON METROPOLITAN AREA LATE THIS MORNING INTO  
EARLY THIS AFTERNOON, THEN TOWARDS BALTIMORE BY LATE THIS  
AFTERNOON. THE HEAVIEST RAIN IS EXPECTED ACROSS CENTRAL VIRGINIA  
INTO THE POTOMAC HIGHLANDS WHERE IT WILL BE RAINING ALL DAY.  
TEMP/DEWPOINT DEPRESSIONS EARLY THIS MORNING REMAIN PRETTY LARGE  
SO EVAPORATIVE COOLING IS EXPECTED ONCE RAIN BEGINS. THEREFORE,  
RAN A NON-DIURNAL TREND FOR THE HOURLY TEMPERATURES AND IT WILL  
TURN OUT TO BE QUITE CHILLY FOR THIS TIME OF YEAR WITH TEMPS  
HOLDING IN THE 40S ACROSS THE MOUNTAINS TO THE 50S ACROSS MOST  
OTHER LOCATIONS. A FEW AREAS IN NORTHEASTERN MARYLAND WILL  
APPROACH 60 WHERE IT WILL TAKE LONGER FOR THE RAIN TO MOVE IN.  
 
THE CUTOFF LOW WILL PASS BY TO THE SOUTH THIS EVENING BEFORE  
GETTING PICKED UP BY THE JETSTREAM AND EVENTUALLY MOVING UP THE  
MID-ATLANTIC COAST OVERNIGHT. OUR AREA WILL REMAIN ON THE COOL  
SIDE OF THE BOUNDARY, AND WITH MORE OVERRUNNING IN PLACE THIS  
WILL ALLOW FOR A SOAKING RAIN FOR MUCH OF THE NIGHT. RAINFALL  
RATES WILL SUBSIDE A BIT AS THE DRY SLOT WORKS ITS WAY IN ALOFT.  
 

 
   
SHORT TERM /WEDNESDAY THROUGH THURSDAY NIGHT/
 
 
THE SURFACE LOW WILL CONTINUE TO MOVE UP THE MID-ATLANTIC COAST  
WEDNESDAY WHILE THE UPPER-LEVEL LOW ASSOCIATED WITH THIS SYSTEM  
PASSES BY TO OUR SOUTH. A COLD FRONT WILL ALSO PASS THROUGH THE  
AREA WEDNESDAY EVENING. UNSETTLED CONDITIONS ARE EXPECTED WITH A  
FEW SHOWERS LIKELY WEDNESDAY DUE TO THE LOW PASSING BY COMBINED  
WITH THE APPROACHING COLD FRONT. HOWEVER, IT DOES NOT APPEAR  
THAT WEDNESDAY WILL BE A WASHOUT SINCE THERE WILL BE BREAKS IN  
THE PRECIPITATION. THERE MAY EVEN BE SOME BREAKS OF SUNSHINE,  
BUT CLOUD SHOULD HANG AROUND FOR MUCH OF THE DAY. WEDNESDAY WILL  
TURN OUT A BIT MILDER AHEAD OF THE COLD FRONT WITH MAX TEMPS IN  
THE 60S FOR MOST AREAS.  
 
DRIER AIR WILL MOVE IN BEHIND THE COLD FRONT WEDNESDAY NIGHT DUE  
TO A NORTHWEST FLOW. HIGH PRESSURE WILL RETURN FOR THURSDAY,  
BRINGING SOME SUNSHINE AND DRY CONDITIONS ALONG WITH SEASONABLE  
TEMPERATURES.  
 
LOW PRESSURE MAY IMPACT THE AREA OVERNIGHT THURSDAY WITH RAIN  
POSSIBLE, BUT CONFIDENCE REMAINS LOW AT THIS TIME AND LARGE  
DIVERGENCE AMONG GUIDANCE AS TO HOW STRONG THE LOW WILL BE AND  
HOW FAR NORTH IT WILL TRACK.  
 

 
   
LONG TERM /FRIDAY THROUGH MONDAY/
 
 
AN AREA OF LOW PRESSURE SHOULD MOVE ACROSS OUR SOUTHEASTERN ZONES  
FRIDAY AND BRING A CHANCE FOR RAIN SHOWERS TO THE REGION. THIS  
CHANCE OF RAIN SHOWERS COULD EXTEND BACK AS FAR AS I-81. THERE  
REMAINS SOME UNCERTAINTY ON THE TRACK OF THE LOW AND THE  
STRENGTH OF THE LOW BETWEEN THE MODELS. FOR NOW, WE WILL KEEP  
THE CHANCE OF RAIN SHOWERS. TEMPERATURES SHOULD REMAIN NEAR TO  
SLIGHTLY BELOW AVERAGE, AND COULD VARY GREATLY DEPENDING ON THE  
TRACK OF THE LOW.  
 
A WEAK COLD FRONT SATURDAY WILL MOVE ACROSS OUR REGION BEFORE  
MIDDAY. A FEW RAIN SHOWERS CANNOT BE RULED OUT ALONG AND AHEAD OF  
THIS FRONT. TEMPERATURES WILL BE CLOSE TO AVERAGE FOR THIS TIME  
IN APRIL.  
 
HIGH PRESSURE BUILDS IN ON SUNDAY AND MONDAY. DRY CONDITIONS AND  
WARMER TEMPERATURES WILL ENSUE EACH DAY.  
 

 
   
AVIATION /15Z TUESDAY THROUGH SATURDAY/
 
 
RAIN WILL OVERSPREAD THE TERMINALS FROM SOUTHWEST TO NORTHEAST  
THIS MORNING INTO THIS AFTERNOON. ALONG WITH THE RAIN WILL BE  
LOWERING CIGS/VSBYS. TIMING IS A BIT UNCERTAIN, BUT IT LOOKS  
LIKE MVFR CONDITIONS WILL OVERSPREAD THE TERMINALS LATE THIS  
MORNING INTO THIS AFTERNOON AND IFR CONDITIONS WILL OVERSPREAD  
THE TERMINALS TONIGHT. HOWEVER, CONDITIONS ARE EXPECTED TO  
DETERIORATE A LITTLE QUICKER FOR KCHO WHERE IFR CONDITIONS ARE  
POSSIBLE THIS AFTERNOON. ALSO, PERIODS OF IFR VSBYS ARE POSSIBLE  
IN BANDS OF MODERATE RAIN, ESPECIALLY THIS AFTERNOON.  
 
LOW CLOUDS ALONG WITH RAIN AND DRIZZLE WILL HANG AROUND FOR  
TONIGHT WITH IFR CONDITIONS EXPECTED. CONDITIONS WILL GRADUALLY  
IMPROVE WEDNESDAY AS LOW PRESSURE PASSES BY TO OUR EAST, BUT  
THERE WILL BE A FEW SHOWERS AROUND. DRIER AIR AND A NORTHWEST  
FLOW ARE EXPECTED WEDNESDAY NIGHT THROUGH THURSDAY WITH VFR  
CONDITIONS. LOW PRESSURE MAY IMPACT THE TERMINALS OVERNIGHT  
THURSDAY BUT CONFIDENCE REMAINS LOW.  
 
SUB-VFR CONDITIONS POSSIBLE FRIDAY WITH A NEARBY LOW PRESSURE  
SYSTEM. VFR CONDITIONS AT ALL TERMINALS FRIDAY NIGHT THROUGH  
SATURDAY NIGHT. WINDS SOUTHWEST AROUND 5 KNOTS FRIDAY AND FRIDAY  
NIGHT. WINDS BECOMING NORTHWEST 5 TO 10 KNOTS SATURDAY AND SATURDAY  
NIGHT.  
 

 
   
MARINE
 
 
AN EASTERLY FLOW WILL GRADUALLY STRENGTHEN THIS MORNING AS LOW  
PRESSURE APPROACHES FROM THE SOUTH. THE LOW WILL MOVE UP THE  
MID-ATLANTIC COAST LATE TONIGHT AND PASS BY TO OUR EAST  
WEDNESDAY. THE GRADIENT WILL SUBSIDE DURING THIS TIME. A  
NORTHWEST FLOW IS EXPECTED ON THE BACK SIDE OF THE LOW WEDNESDAY  
NIGHT INTO THURSDAY. A SMALL CRAFT ADVISORY IS IN EFFECT FOR THE  
WATERS TODAY INTO THIS EVENING, AND FOR THE BAY AND LOWER TIDAL  
POTOMAC RIVER INTO LATE TONIGHT. WILL CONTINUE WITH A LULL IN  
THE WINDS FOR WEDNESDAY AS THE LOW PASSES BY TO THE EAST, THEN A  
SMALL CRAFT ADVISORY WILL LIKELY BE NEEDED WEDNESDAY NIGHT AND  
THURSDAY FOR PORTIONS OF THE WATERS.  
 
LOW PRESSURE MAY IMPACT THE WATERS LATER THURSDAY NIGHT BUT  
CONFIDENCE REMAINS LOW AT THIS TIME.  
 
NO MARINE HAZARDS EXPECTED FRIDAY THROUGH SATURDAY NIGHT. WINDS  
SOUTHWEST 5 TO 10 KNOTS FRIDAY AND FRIDAY NIGHT. WINDS BECOMING  
NORTHWEST 10 KNOTS SATURDAY AND SATURDAY NIGHT.  
 

 
   
HYDROLOGY
 
 
WIDESPREAD RAINFALL AMOUNTS FROM 1 TO 2 INCHES ARE FORECAST  
THROUGH WEDNESDAY, WITH LOCALLY HIGHER AMOUNTS LIKELY ALONG THE  
CENTRAL BLUE RIDGE AND ADJACENT FOOTHILLS TO THE EAST. DUE TO  
RELATIVELY DRY ANTECEDENT CONDITIONS AND EXTENDED TIME PERIOD  
OVER WHICH THE RAIN FALLS, WIDESPREAD HYDROLOGICAL CONCERNS ARE  
NOT ANTICIPATED. HOWEVER, WITH INCREASING SOUTHEAST FLOW,  
LOCALIZED AMOUNTS OF UP TO 3" ARE POSSIBLE, AGAIN MAINLY NEAR  
THE HIGHER ELEVATIONS OF THE BLUE RIDGE AND NEAR-BY HIGHLANDS  
WHICH COULD LEAD TO SOME MINOR HYDROLOGY ISSUES.  
 

 
   
TIDES/COASTAL FLOODING
 
 
AN EASTERLY FLOW WILL STRENGTHEN TODAY AS LOW PRESSURE  
APPROACHES FROM THE SOUTH. THE EASTERLY FLOW WILL CONTINUE  
TONIGHT BEFORE BECOMING LIGHT WEDNESDAY AND TURNING NORTHWEST  
WEDNESDAY NIGHT.  
 
TIDAL ANOMALIES WILL CONTINUE TO INCREASE TODAY. THERE IS A  
BETTER CHANCE FOR MINOR TIDAL FLOODING WITH THE HIGH TIDE CYCLE  
TONIGHT INTO WEDNESDAY MORNING MAINLY AT STRAITS POINT, DESPITE  
IT BEING THE LOWER OF THE TWO ASTRONOMICAL NORMS. WATER LEVELS  
SHOULD SUBSIDE LATER WEDNESDAY NIGHT WITH THE NORTHWEST FLOW.  
 

 
   
LWX WATCHES/WARNINGS/ADVISORIES
 
 
DC...NONE.  
MD...NONE.  
VA...NONE.  
WV...NONE.  
MARINE...SMALL CRAFT ADVISORY UNTIL 4 AM EDT WEDNESDAY FOR ANZ530-531-  
538>540-542.  
SMALL CRAFT ADVISORY UNTIL 4 AM EDT WEDNESDAY FOR ANZ532>534-  
537-541-543.  
SMALL CRAFT ADVISORY UNTIL 10 PM EDT THIS EVENING FOR ANZ535-  
536.  
 

 
 

 
SYNOPSIS...BJL  
NEAR TERM...LFR  
SHORT TERM...BJL  
LONG TERM...KLW  
AVIATION...BJL/KLW  
MARINE...BJL/KLW  
HYDROLOGY...BJL  
TIDES/COASTAL FLOODING...LFR  
 
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