400 FXUS02 KWBC 171816 PMDEPD EXTENDED FORECAST DISCUSSION FOR 20 OCT THRU 22 OCT 1998 HYDROMETEOROLOGICAL PREDICTION CENTER..NCEP...NWS...WASHINGTON DC 2:15 PM EDT SAT 17 OCT 1998 ...MODEL DISCUSSION AND OVERVIEW... LATEST MEDIUM-RANGE MODEL D+3 500MB MEANS CONTINUE TO FORECAST A MAJOR LONGWAVE CHANGE OVER THE ERN PACIFIC AND NOAM. A NEAR REVERSAL OF THE RECENT E CENTRAL PACIFIC RIDGE/W COAST TROF PATTERN IS DUE. HEIGHTS ARE FORECAST TO FALL STRONGLY NEAR 45N/160W...AND RISE STRONGLY OVER THE PACIFIC NW/VANCOUVER/BC. SIGNIFICANT HEIGHT FALLS ARE ALSO EXPECTED OVER THE GREAT LAKES IN RESPONSE TO THE UPSTREAM AMPLIFICATION. A PIECE OF LEFTOVER VORT WILL BE TRAPPED UNDERNEATH THE STRONG RIDGE IN THE PACIFIC NW STATES...GENERATING SW FLOW ALOFT THAT WILL OVERRUN A COOL DOME OF SFC HIGH PRES IN TX/ERN NM MON- TUE. HIGH LEVEL MOISTURE FROM MADELINE MAY ENHANCE OVERRUNNING PCPN IN SWRN TX AND SERN NM. THE MOST IMPORTANT MODEL DIFFERENCES TODAY CENTER ON THE DETAILS OF THE AMPLIFYING MEAN TROF OVER THE GREAT LAKES AREA WED-THU. FOR WED...THE ECMWF/NAVY NOGAPS ARE STRONGEST WITH THIS NEW TROF AND DROP A VERY STRONG VORT MAX ACROSS THE UPPER MIDWEST AND MID MS VLY...RESPECTIVELY. BY FRI...THE NAVY NOGAPS/UKMET ARE THE STRONGEST OF THE MODELS...SHOWING A DEEP ELONGATED TROF/CLOSED LOW FROM THE LOWER OH VLY TO THE LOWER LAKES. THE ECMWF/MRF FOR THU DO NOT DIG THE UPPER TROF AS FAR S AS THE NOGAPS/UKMET...SHOWING THEIR HIGHEST VORT VALUES FROM THE OH VLY TO THE MID-ATLANTIC. A BLEND OF MODEL FORECASTS FOR WED-THU IS PROBABLY OUR COURSE OF LEAST REGRET. WE WONT BE ABLE TO REFINE THE DETAILS OF THIS DIGGING SHORTWAVE... UNTIL IT DROPS DOWN THE E SIDE OF THE BUILDING LONGWAVE RIDGE ALONG THE W COAST OF CANADA IN ABOUT TWO DAYS. ....REGIONAL HIGHLIGHTS.... ...SWRN STATES/TX... A COOL DOME OF HIGH PRES IN TX WILL BE OVERRUN BY SW FLOW ALOFT...WHICH MAY BE TAPPING MOISTURE FROM TROPICAL SYSTEMS MADELINE OR EVEN LESTER ON THE PACIFIC SIDE OF THE MEXICAN COAST. SOME SOAKING LOCALLY HEAVY RAINFALL IS POSSIBLE MON-TUE IN SW TX/SERN NM...MORE FROM A PROLONGED PERIOD OF MODERATE STRATIFORM RAIN THAN FROM INTENSE CONVECTIVE DOWNPOURS. THE ECMWF/MRF LEAVED A WEAK CLOSED UPPER LOW TRAPPED VCNTY OF SRN CA/AZ THRU THE MEDIUM RANGE. THE NAVY NOGAPS/ECMWF WEAKEN THIS CLOSED UPPER LOW LATE WED INTO THU...MOVING IT NWD INTO NV. WE DO EXPECT SOME DRIER AIR TO THE REAR OF THE DIGGING UPPER TROF IN THE MID/UPPER MS VLY WED-THU TO FILTER S INTO TX/ERN NM...BRINGING A GRADUAL END TO PCPN BY THU. THE NAVY NOGAPS/UKMET...DIGGING THIS TROF FARTHER S INTO THE WRN GULF STATES THAN THE MRF/ECMWF...IMPLY MORE RAPID DRYING FOR TX/NM...WITH PCPN ENDING WED. ...REMAINDER OF THE WRN HALF OF THE COUNTRY.. THIS PORTION OF THE NATION WILL HAVE MOSTLY DRY WEATHER UNDER THE INFLUENCE OF THE UPPER RIDGE. TEMPS WILL START OUT BELOW NORMAL BUT MODERATE WITH TIME. THE PACIFIC COAST MAY SEE WELL ABOVE NORMAL MAXIMA BY WED AS SURFACE PRES BUILD INTO THE GREAT BASIN AND STRONG SANTA ANNA GRADIENTS DEVELOP. ...ERN HALF OF THE US... TEMPERATURES WILL GET COLDER WITH TIME AS UPPER HEIGHTS FALL OVER THE GREAT LAKES. THE PCPN FORECAST IS PROBLEMATIC...DEPENDING ON HOW MUCH HEIGHTS FALL OVER THE LAKES/APLCNS BY THE END OF THE PERIOD. A CONSENSUS OF MODELS SHOWS THICKNESSES FALLING TO BELOW 5400M OVER THE GREAT LAKES LATER TUE. THIS WILL GENERATE SOME INSTABILITY SHOWERS...BUT BECAUSE OF THE WARM LAKES...THESE SHOULD BE RAIN RATHER THAN SNOW. DEPENDING ON THE AMPLITUDE AND SRN EXTENT OF THE GREAT LAKES TROF...PCPN MAY DEVELOP IN THE MID ATLANTIC-NEW ENG AREAS BY THU. THE ECMWF AND NAVY NOGAPS BEST LEAD INTO A COASTAL CYCLOGENESIS SCENARIO...SINCE THEY ALREADY HAVE A SURFACE LOW IN THE ERN LAKES WED. SOME SHOWERS MAY LINGER OVER PENINSULAR FL THRU THE PERIOD. FLOOD/FORECAST OPERATIONS BRANCH GRAPHICS AVAILABLE ON THE WEB AT http://www.ncep.noaa.gov/HPC