Issued by NWS Spokane, WA
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000 FXUS66 KOTX 211132 AFDOTX AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE SPOKANE WA 330 AM PST SAT NOV 21 2009 .SYNOPSIS... AFTER A SHORT RELATIVELY QUIET BREAK IN THE WEATHER TODAY...A STRONGER AND COLDER STORM SYSTEM WILL BRING THE POTENTIAL FOR MODERATE TO HEAVY SNOWFALL OVER MUCH OF THE REGION TONIGHT AND SUNDAY. A SERIES OF STORM SYSTEMS WILL CONTINUE TO PASS THROUGH THE INLAND NORTHWEST EVERY 24 TO 36 HOURS THROUGH AT LEAST THE MIDDLE OF NEXT WEEK. && .DISCUSSION... TODAY...WHILE THIS MORNING`S SATELLITE LOOP LOOKS MESSY...IN GENERAL THE FORECAST AREA IS CURRENTLY UNDER AN INTER TROUGH COLD POOL ALOFT WHICH WILL EXIT THE REGION AND ALLOW A DIRTY AND WEAK SHORT WAVE RIDGE TO SET UP OVER THE REGION FOR THIS AFTERNOON. IN THE WAKE OF LAST EVENINGS COLD FRONT PASSAGE BREEZY WINDS WILL CONTINUE THROUGH THE MORNING AS A COOLER AIR MASS FILLS THE BASIN. THERE WILL LIKELY BE SOME RESIDUAL SNOW SHOWERS MAINLY CONCENTRATED OVER THE IDAHO PANHANDLE MOUNTAINS AND NEAR THE CASCADE CREST...WITH PLENTY OF CLOUDS AND A SMALL CHANCE OF A STRAY VALLEY RAIN OR SNOW SHOWER POSSIBLE OVER THE RISING TERRAIN OF THE EASTERN BASIN TODAY AS A MOIST WESTERLY FLOW ALOFT CONTINUES. HIGH TEMPERATURES TODAY WILL HAVE TROUBLE BREAKING OUT OF THE UPPER THIRTIES AT MOST LOCATIONS OTHER THAN THE LOWEST VALLEYS. TONIGHT THROUGH SUNDAY...A VERY TRICKY FORECAST FOR THE NEXT 24 TO 36 HOURS. THE NEXT STORM SYSTEM...VISIBLE ON PACIFIC SATELLITE AS A DEVELOPING BAROCLINIC LEAF STRUCTURE APPROACHING 140W THIS MORNING...WILL MOVE ASHORE SOMETIME THIS EVENING AND CROSS THE FORECAST AREA OVERNIGHT AND INTO SUNDAY MORNING. THERE ARE SOME SIGNIFICANT MODEL DIFFERENCES WITH THE TRACK OF THE DEVELOPING SURFACE LOW AND THE STRENGTH OF THE PARENT TROUGH. ALL THE MODELS DEVELOP A STRONG SURFACE LOW JUST OFF THE COAST THIS AFTERNOON AND EJECT IT INLAND TONIGHT. THE GFS AND CANADIAN MODELS TRACK THE LOW THROUGH THE CENTRAL CASCADES AND SWING IT NORTH INTO BRITISH COLUMBIA...WHILE THE ECMWF AND NAM GUIDANCE TRACK THE LOW THROUGH THE SOUTHERN COLUMBIA BASIN. THIS LOW TRACK IS CRITICAL FOR SNOW AMOUNT POTENTIAL. ALTHOUGH ALL MODELS INDICATE SIGNIFICANT ACCUMULATING SNOW IS LIKELY OVERNIGHT SATURDAY AND SUNDAY MORNING...A MORE SOUTHERLY TRACK WILL BRING HEAVIER SNOW AMOUNTS TO THE VALLEYS AND BASIN. THUS CONFIDENCE IS HIGH THAT MOST OF THE FORECAST AREA NORTH OF INTERSTATE 90 WILL EXPERIENCE AT LEAST A COUPLE INCHES OF SNOW TONIGHT AND SUNDAY MORNING...WITH MODERATE TO HEAVY ACCUMULATIONS IN THE MOUNTAINS. CONFIDENCE IS LOW AT THIS TIME WHICH MODEL WILL VERIFY BEST...AND THUS UNCERTAINTY REMAINS ABOUT THE POTENTIAL FOR HIGHLIGHTS WITH THIS SYSTEM. THUS A SLEW OF WATCHES HAVE BEEN ISSUED...AND HOPEFULLY FURTHER MODEL RUNS TODAY WILL SETTLE ON A CONFIDENT SOLUTION. AT THIS TIME WE HAVE OPTED FOR A TRACK THAT IS SLIGHTLY SOUTH OF THE GFS/CANADIAN MODELS AND DELAYED THE TIMING CLOSER TO THE EC/NAM GIVEN THE SATELLITE EXTRAPOLATION AND AGGRESSIVE DIGGING OF THE PARENT TROUGH. THIS ARGUES FOR AN ONSET OF LIGHT TO MODERATE SNOW IN THE WESTERN ZONES THIS EVENING...INTENSIFYING OVERNIGHT THEN CUTTING OUT/TAPERING OFF WITH THE COLD FRONT PASSAGE SUNDAY MORNING. ONSET OVER THE EAST AND NORTHERN ZONES WILL HOLD OFF UNTIL LATE OVERNIGHT SATURDAY OR EARLY SUNDAY MORNING...REACH MAXIMUM INTENSITY SUNDAY MORNING AND TAPER OFF SUNDAY AFTERNOON WITH THE USUAL LINGERING DENSE SNOW SHOWERS OVER THE IDAHO PANHANDLE. /FUGAZZI SUNDAY NIGHT THROUGH FRIDAY...THE WEEKEND STORM SYSTEM CONTINUES ITS EXIT TO THE EAST SUNDAY NIGHT ALLOWING FOR THE HIGHER POPS ASSOCIATED WITH IT TO SHOW A SIMILAR EXIT OUT TO THE EAST SUNDAY NIGHT AS WEAK RIDGING MOVES THROUGH BEHIND IT. THIS RIDGING IS NOT WELL AMPLIFIED AND APPEARS PRONE TO OVERTOPPING BY TWO STORM SYSTEMS. THE FIRST STORM SYSTEM DRAGS A WELL MAINTAINED MOISTURE TAPS THROUGH EASTERN WASHINGTON AND NORTHERN IDAHO MONDAY NIGHT INTO TUESDAY WHICH DEFLATES THE WEAK RIDGING FURTHER. THIS ALLOW THE SECOND STORM SYSTEM TO EXERT SOME INFLUENCE AS EARLY AS LATE IN THE DAY TUESDAY...HOWEVER THE RIDGE APPEARS TO SLOWLY AMPLIFY AS A FEW SHORTWAVES START TO DIG ON THE WEST SIDE/NORTHS SIDE OF THE JET OF THE LARGE BAROCLINIC BAND/MOISTURE TAP ASSOCIATED WITH THIS SECOND SYSTEM. THE RESULT IS A POSSIBLY SHUNTING OF ANY PRECIP BAN FRONTAL FEATURE BACK UP TO THE NORTH AND WEST FROM TUESDAY NIGHT TO AS LATE AS THANKSGIVING. EVENTUALLY A DIGGING SHORTWAVE GETS ENOUGH SIGNIFICANT CYCLOGENESIS GOING ON THE BAROCLINIC BAND OFFSHORE TO GET IT WRAPPED UP INTO A COMPACT LOW THAT DUMBBELLS AROUND WITH ANOTHER LOW TO ITS NORTH AS IT MOVES INLAND SOMETIME BETWEEN THE PERIOD BETWEEN THANKSGIVING AND FRIDAY. /PELATTI && .AVIATION... BREEZY CONDITIONS IN THE WAKE OF A COLD FRONT PASSAGE LAST NIGHT WILL KEEP GENERALLY VFR VISIBILITIES AT ALL TAF SITES THIS MORNING...WITH SOME OCCASIONAL MVFR AND IFR STRATUS CEILINGS AT KGEG/KSFF AND KCOE UNTIL 16Z. A WEAK RIDGE ALOFT WILL PROMOTE VFR CONDITIONS AT ALL TAF SITES THROUGH 03Z TONIGHT...WITH LINGERING SNOW SHOWERS LOCALLY OBSCURING MOUNTAINS IN THE IDAHO PANHANDLE. THE NEXT PACIFIC STORM SYSTEM WILL SPREAD MVFR AND LOCAL IFR CIELINGS TO THE WESTERN REACHES OF THE REGION AFTER 03Z TONIGHT...MAINLY IMPACTING THE KEAT AND KMWH TAF SITES. CONDITIONS AT EASTERN TAF SITES WILL BEGIN TO DETERIORATE BY THE END OF THE TAF PERIOD AT 12Z SUNDAY. && .PRELIMINARY POINT TEMPS/POPS... SPOKANE 39 32 37 27 39 31 / 20 70 90 10 40 10 COEUR D`ALENE 39 31 37 28 40 31 / 20 60 90 20 50 20 PULLMAN 40 32 36 28 42 32 / 20 60 90 20 40 20 LEWISTON 47 36 41 30 48 35 / 10 60 90 20 30 20 COLVILLE 39 33 40 28 38 30 / 30 100 90 10 60 20 SANDPOINT 33 32 37 28 36 30 / 40 90 90 20 60 30 KELLOGG 32 30 34 27 35 30 / 70 70 100 40 60 40 MOSES LAKE 45 32 43 26 43 30 / 10 80 50 0 40 10 WENATCHEE 43 32 43 29 41 33 / 30 90 50 10 50 10 OMAK 43 31 42 25 40 27 / 40 100 50 0 60 10 && .OTX WATCHES/WARNINGS/ADVISORIES... ID... WINTER STORM WATCH FROM LATE TONIGHT THROUGH SUNDAY MORNING FOR COEUR D`ALENE AREA. WINTER STORM WATCH FROM THIS EVENING THROUGH SUNDAY MORNING FOR NORTHERN PANHANDLE. WINTER STORM WATCH FROM THIS EVENING THROUGH SUNDAY AFTERNOON ABOVE 2500 FEET FOR CENTRAL PANHANDLE MOUNTAINS. WA... WINTER STORM WATCH FROM LATE TONIGHT THROUGH SUNDAY MORNING FOR SPOKANE AREA. WINTER STORM WATCH FROM THIS EVENING THROUGH SUNDAY MORNING FOR EAST SLOPES NORTHERN CASCADES. WINTER STORM WATCH FROM THIS EVENING THROUGH SUNDAY MORNING ABOVE 2000 FEET FOR NORTHEAST MOUNTAINS...OKANOGAN HIGHLANDS. WINTER STORM WATCH FROM THIS EVENING THROUGH SUNDAY MORNING ABOVE 2000 FEET FOR OKANOGAN VALLEY...UPPER COLUMBIA BASIN...WATERVILLE PLATEAU. && $$
