871  
FXUS61 KLWX 221859  
AFDLWX  
 
AREA FORECAST DISCUSSION  
NATIONAL WEATHER SERVICE BALTIMORE MD/WASHINGTON DC  
259 PM EDT SUN APR 22 2018  
   
SYNOPSIS
 
 
HIGH PRESSURE OVER THE GREAT LAKES WILL SLOWLY MOVE EAST INTO  
THE NORTHEAST U.S. TONIGHT. THIS HIGH WILL MOVE OFFSHORE  
MONDAY. A LOW PRESSURE SYSTEM CURRENTLY OVER THE LOWER  
MISSISSIPPI VALLEY WILL BRING UNSETTLED WEATHER TO THE REGION  
MONDAY NIGHT THROUGH WEDNESDAY. ANOTHER STORM SYSTEM MAY IMPACT  
THE REGION AT THE END OF THE WEEK.  
 

 
   
NEAR TERM /THROUGH MONDAY/
 
 
LATEST SURFACE ANALYSIS DEPICTS HIGH PRESSURE CENTERED OVER THE  
EASTERN GREAT LAKES, WITH AN EXTENSION SOUTHEAST TO A SECOND  
CENTER EAST OF NORFOLK IN THE WESTERN ATLANTIC. THERE IS A WEAK  
SURFACE TROUGH EXTENDING OVER THE REGION BETWEEN THESE TWO  
CENTERS, AND THAT IS HELPING FOCUS A BIT OF CLOUD DEVELOPMENT  
THIS AFTERNOON. THERE IS ALSO AN UPPER LEVEL TROUGH CROSSING THE  
REGION. THANKS TO THE FORCING FROM THESE TWO FEATURES, ITS  
STILL NOT IMPOSSIBLE A FEW SPRINKLES OR A SHOWER POP UP, MOST  
LIKELY OVER WESTERN VA/EASTERN WV, BUT ODDS ARE VERY LOW, LESS  
THAN 25 PERCENT. OTHERWISE, DRY WEATHER WILL BE THE RULE FOR THE  
REMAINDER OF TODAY AND TONIGHT. IN FACT, AS THE TROUGH PASSES  
SOUTHEAST AWAY FROM THE REGION, CLOUDS SHOULD GENERALLY DECREASE  
OVERNIGHT, BUT WITH DEW POINTS NOW COMFORTABLY ABOVE FREEZING,  
WE SHOULD ALSO STAY COMFORTABLY ABOVE FREEZING, WITH LOWS MOSTLY  
IN THE 40S.  
 
ON MONDAY, THE HIGH PRESSURE SYSTEM WILL SHIFT EASTWARD OFF THE  
COAST OF NEW ENGLAND, WHILE LOW PRESSURE OVER THE LOWER  
MISSISSIPPI VALLEY AT PRESENT WILL MIGRATE NORTHEAST TOWARDS  
CENTRAL TENNESSEE. AS THIS HAPPENS, ISENTROPIC LIFT AND MOISTURE  
ADVECTION WILL INCREASE WITH CONTINUED SOUTHEAST FLOW FROM THE  
ATLANTIC, AND RAIN IS LIKELY TO START BREAKING OUT FROM  
SOUTHWEST TO NORTHEAST OVER WEST CENTRAL VIRGINIA. HOWEVER,  
MOST OF THE REGION SHOULD REMAIN DRY THROUGH SUNSET, BUT WITH  
INCREASING CLOUDS. DESPITE THE SOUTHEASTERLY FLOW, TEMPERATURES  
SHOULD GENERALLY REMAIN AS MILD AS TODAY, OR PERHAPS MODERATE A  
TAD FURTHER, WITH 60S COMMON. COOLEST READINGS WILL BE IN WEST-  
CENTRAL VA WHERE GREATER CLOUD COVER AND THE THREAT OF LATE-DAY  
RAIN WILL HOLD TEMPS DOWN.  
 

 
   
SHORT TERM /MONDAY NIGHT THROUGH TUESDAY NIGHT/
 
 
CLOSED LOW OVER WESTERN TENNESSEE EARLY MONDAY NIGHT WILL  
MIGRATE EAST AND NORTH INTO KENTUCKY THROUGH TUESDAY, FINALLY  
OPENING BACK UP INTO A SHORTWAVE TROUGH TUESDAY NIGHT AS IT GETS  
PUNTED EASTWARD MORE RAPIDLY THANKS TO ANOTHER SHORTWAVE DROPING  
SOUTHEAST ACROSS THE NORTHERN PLAINS. IN RESPONSE TO THIS,  
SURFACE LOW PRESSURE OVER THE TENNESSEE VALLEY WILL START TO  
TRANSFER TO A NEW LOW IN THE CAROLINAS ON TUESDAY. THIS SECOND  
LOW WILL MOVE NORTHEASTWARD UP THE COAST TUESDAY NIGHT AND  
WEDNESDAY. ISENTROPIC LIFT, UPPER DIVERGENCE, FGEN AND MOISTURE  
ADVECTION, COMBINED WITH UPSLOPE LOW LEVEL FLOW FROM THE  
SOUTHEAST, SHOULD PROVIDE ADEQUATE FORCING FOR A DECENT DOSE OF  
RAINFALL. THE QUESTION WHICH WE ARE STUCK WITH IS: JUST HOW  
HEAVY? MANY AREAS HAD PRETTY SIGNIFICANT RAIN LAST MONDAY, BUT  
IT HAS NOW BEEN ABOUT A WEEK, AND WILL BE 8 DAYS PAST ON  
TUESDAY, SO WHILE SOILS ARE STILL MORE MOIST THAN THEY WERE  
BEFORE MONDAY'S RAIN, THEY HAVE DRIED OUT A BIT, AND RIVERS HAVE  
ALSO GONE DOWN. RIGHT NOW, GUIDANCE IS FLUCTUATING A BIT WITH  
REGARDS TO HOW MUCH RAIN WILL FALL, BUT BEST ESTIMATE PLACES  
MOST OF THE REGION IN THE 1-2 INCH RANGE. LOCALLY HIGHER AMOUNTS  
MAY RESULT IN ISOLATED FLOODING, BUT CONFIDENCE IS NOT HIGH  
SINCE THE SURFACE LOW SHOULD STAY SOUTHEAST OF THE REGION,  
POTENTIALLY LIMITING THE CONVECTIVE ELEMENTS. THUS, WILL HOLD  
OFF ON POSTING ANY MENTION OF FLOODING IN THE HWO FOR THE TIME  
BEING, THOUGH IT IS NOTED THAT WPC HAS PLACED THE REGION IN A  
MARGINAL RISK OF EXCESSIVE RAINFALL.  
 
WITH THE PASSAGE OF THE LOW PRESSURE AND ACCOMPANYING CLOUDS AND  
RAIN, TEMPERATURES MONDAY NIGHT THROUGH TUESDAY NIGHT SHOULD NOT  
FLUCTUATE MUCH, WITH 40S FOR LOWS MONDAY NIGHT, AND TEMPERATURES  
MOSTLY IN THE 50S TUESDAY AND TUESDAY NIGHT.  
 

 
   
LONG TERM /WEDNESDAY THROUGH SUNDAY/
 
 
LOW PRESSURE WILL BE MOVING NORTHEAST AWAY FROM THE AREA  
WEDNESDAY MORNING, LEAVING A CHANCE FOR SOME RESIDUAL SHOWERS IN  
ITS WAKE. ANOTHER QUICK MOVING SHORWAVE WILL MOVE THROUGH  
WEDNESDAY EVENING INTO THURSDAY MORNING. SOME SHOWERS WILL BE  
POSSIBLE WITH THIS SYSTEM AS WELL, ESPECIALLY WEDNESDAY NIGHT.  
TEMPERATURES WEDNESDAY WILL BE BELOW AVERAGE, THANKS TO THE  
CLOUD COVER AND PRECIPITATION EXPECTED OVER THE AREA.  
 
WEAK HIGH PRESSURE WILL MOVE INTO THE REGION IN THE WAKE OF  
THIS SYSTEM ON THURSDAY INTO THURSDAY NIGHT, BRINGING DRIER  
WEATHER INTO THE FORECAST AGAIN, BUT TEMPERATURES REMAIN BELOW  
NORMAL.  
 
A LOT OF UNCERTAINTY FOR FRIDAY'S FORECAST AT THIS TIME. ONE  
THING THAT SEEMS CONSISTENT AMONG ALL GUIDANCE, IS THAT A  
SHORTWAVE WILL MOVE ACROSS THE SOUTHEAST LATE THURSDAY INTO  
EARLY FRIDAY. THIS SHORTWAVE SPAWNS A SURFACE LOW SOMEWHERE IN  
THE SOUTHEAST IN THAT SAME TIME PERIOD. THE UNCERTAINTY COMES  
WITH DETERMINING WHERE EXACTLY THIS SYSTEM MOVES, AND HOW STRONG  
IT WILL BE. MODEL GUIDANCE HAS BEEN VERY JUMPY WITH THIS  
SYSTEM, THUS NOT GIVING A CLEAR PATTERN TO RUN WITH. ON ONE  
HAND, YOU HAVE THE EUROPEAN, WHICH SLIDES THIS SYSTEM OFF THE  
COAST AND KEEPS PRECIPITATION CHANCES HERE VERY LOW. HOWEVER,  
BOTH THE CANADIAN AND GFS WANT TO BRING THE LOW UP THE COAST,  
BRINGING PRECIPITATION CHANCES TO THE AREA ON FRIDAY. THESE  
MODELS DIFFER FROM ONE ANOTHER IN TERMS OF STRENGTH AS WELL,  
WITH THE GFS PHASING THE SHORTWAVE WITH THE MAIN TROUGH AXIS,  
THUS BRINGING A STRONGER SYSTEM TO THE AREA, WHILE THE CANADIAN  
DOES NOT PHASE THE SYSTEMS, RESULTING IN A WEAKER COASTAL LOW,  
AND LESSS PRECIPITATION. THE GFS HAD BEEN MORE ALLIGNED WITH THE  
EUROPEAN THE PAST FEW RUNS, SO GOING TO GO WITH A MORE  
INTERMEDIATE APPROACH AND SAY THAT WE WILL SEE SOME IMPACT FROM  
A COASTAL LOW, BUT NOT THINKING IT WILL BE AS STRONG AS THE  
LATEST GFS SUGGESTS. THE BOTTOME LINE IS, PRECIPIATION IS  
POSSIBLE FRIDAY AND TEMPERATURES WILL GREATLY DEPEND ON THE  
EVOLUTION OF THE AFORMENTIONED SURFACE LOW, BUT THINKING WE WILL  
BE BELOW AVERAGE ONCE AGAIN.  
 
ANOTHER LOW PRESSURE SYSTEM LOOKS TO SWING THROUGH AREAS ALONG  
THE CANADIAN/NORTHEASTERN U.S. BORDER ON FRIDAY NIGHT INTO  
SATURDAY. A COLD FRONT ASSOCIATED WITH THIS SYSTEM LOOKS TO MOVE  
THROUGH THE AREA ON SATURDAY, BRINGING ANOTHER CHANCE FOR  
PRECIPITATION. MODEL GUIDANCE HAS BACKED OFF A BIT ON THE  
STRENGTH OF THS FRONT, BUT SOME LIGHT PRECIPITATION IS CERTAINLY  
POSSIBLE. THE TIME OF FRONTAL PASSAGE WILL DETERMINE HOW WARM  
WE CAN MANAGE TO GET ON SATURDAY, BUT WITH LINGERING CLOUDS FROM  
FRIDAYS SYSTEM, AND CLUODS ASSOCIATED WITH THE AFORMENTIONED  
FRONT, TEMPERATUERS WILL PROBABLY REMAIN AT, OR SLIGHTLY BELOW,  
AVERAGE.  
 

 
   
AVIATION /18Z SUNDAY THROUGH FRIDAY/
 
 
VFR ALL TERMANALS THROUGH LATE MONDAY, THOUGH CIGS MAY START  
LOWERING TOWARDS MVFR ACROSS THE REGION BY THEN, ESPECIALLY AT  
CHO. WINDS MOSTLY FROM THE SOUTHEAST, BUT LIGHT UNTIL MONDAY  
AFTERNOON, WHEN THE GRADIENT BETWEEN THE HIGH DEPARTING AND THE  
LOW APPROACHING WILL INCREASE THE SOUTHWESTERLY FLOW TO 10-20  
KNOTS.  
 
SUB-VFR LIKELY BY LATE MONDAY NIGHT THROUGH TUESDAY NIGHT ALL  
TERMINALS, WITH INTERVALS OF IFR POSSIBLE THANKS TO RAIN, MIST  
AND LOW CIGS. WINDS SHIFTING FROM SOUTHEAST TO NORTH BY LATE  
TUESDAY NIGHT.  
 
CLOUDS WILL STICK AROUND FOR MUCH OF THE DAY ON WEDNESDAY SUB-  
VFR CONDIONTS POSSIBLE DURUING THIS TIME. CONDITIONS SHOULD  
CLEAR TO VFR BY THURSDAY DURING THE LATE MORNIGN TO EARLY  
AFTERNOON HOURS.  
 

 
   
MARINE
 
 
WINDS MOSTLY SUB SCA THRU MONDAY MORNING, THOUGH SOME GUSTS TO  
AROUND 20 KNOTS THANKS TO SOUTHEASTERLY CHANNELLING ARE  
OCCURRING AT PRESENT OVER THE LOWER AND MIDDLE POTOMAC. DON'T  
THINK THEY LAST BEYOND THE NEXT TWO HOURS, BUT THEY COULD  
APPROACH SCA LIMITS. OVERNIGHT, THE WINDS THERE SHOULD  
DEFINITELY RELAX.  
 
SCA UP FOR LOWER POTOMAC AND MIDDLE CHESAPEAKE ON MONDAY  
AFTERNOON AND NIGHT AS SOUTHEASTERLY FLOW AHEAD OF APPROACHING  
LOW INCREASES. THIS COULD ALSO CAUSE CHANNELING WHICH MIGHT  
RESULT IN SCA FURTHER NORTH INTO THE MIDDLE POTOMAC, BUT WILL  
HOLD ON THAT FOR NOW. SCA MAY EXPAND TO ALL ZONES ON TUESDAY AS  
LOW APPROACHES AND WINDS SHIFT TO EAST AND THEN PERHAPS  
NORTHEAST, FINALLY TURNING NORTH AND DIMINISHING LATER TUESDAY  
NIGHT AS THE LOW PASSES. NOTE THAT THE SOUTHEASTERN WATERS WE  
COVER, MOST LIKELY THE MIDDLE BAY, COULD ACTUALLY BECOME  
SOUTHERLY IF THE LOW PASSES TO THE NORTH OF THOSE AREAS. RAIN  
AND FOG WILL BE A CONCERN AS WELL DURING THIS PERIOD.  
 
AS THE STORM SYSTEM FROM TUESDAY EXITS WEDNESDAY MORNING, SOME  
GUSTY WINDS ARE POSSIBLE ACROSS THE MARINE AREAS, WITH SCA  
CRITERIA WINDS POSSIBLE DURING THAT TIME. GUSTY WINDS ALSO  
POSSIBLE THURSDAY MORNING, BUT NOT LOOKING AS STRONG AS ON  
WEDNESDAY AT THIS TIME.  
 

 
   
HYDROLOGY
 
 
WHILE WE HAD SIGNIFICANT RAIN A WEEK AGO, ITS BEEN PRETTY DRY  
SINCE THEN. THAT SAID, 1-2 INCHES OF RAIN LOOKS LIKELY ACROSS  
THE REGION MONDAY NIGHT THROUGH WEDNESDAY, WHICH COULD  
EXACERBATE HYDROLOGY ISSUES AND CAUSE SIGNIFICANT RIVER AND  
STREAM RISES, POTENTIALLY UP TO FLOOD STAGES. WILL BE CLOSELY  
MONITORING THIS FOR POTENTIAL NEED OF ANY FLOOD WATCHES OVER THE  
NEXT DAY OR SO.  
 

 
   
TIDES/COASTAL FLOODING
 
 
AS LOW PRESSURE APPROACHES THE WATERS FROM THE SOUTHEASTERN  
U.S. LATE MONDAY INTO TUESDAY, A PERSISTENT ONSHORE FLOW WILL  
DEVELOP. THIS WILL YIELD INCREASING TIDAL ANOMALIES, AND THE  
POSSIBILITY OF COASTAL FLOODING TOWARD THE MIDDLE PART OF THE  
WORK WEEK.  
 

 
   
LWX WATCHES/WARNINGS/ADVISORIES
 
 
DC...NONE.  
MD...NONE.  
VA...NONE.  
WV...NONE.  
MARINE...SMALL CRAFT ADVISORY FROM 1 PM MONDAY TO 6 AM EDT TUESDAY FOR  
ANZ534-537-543.  
 

 
 

 
SYNOPSIS...RCM  
NEAR TERM...RCM  
SHORT TERM...RCM  
LONG TERM...CJL  
AVIATION...RCM/CJL  
MARINE...RCM/CJL  
HYDROLOGY...RCM  
TIDES/COASTAL FLOODING...RCM  
 
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