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National Weather Service meteorologists use a wide variety of tools such as observations, satellite imagery, radar imagery, and computer models to aid in making forecasts. The Enhanced Area Forecast Discussion is intended to provide additional insight into our traditional Area Forecast Discussion (AFD) by including embedded links to some of the key tools used in the forecast process. These links are specific to the forecast issue of the day and are updated as the forecast discussion is updated. For comments or feedback please contact the Seattle WFO Webmaster For more information on the terms used in the Enhanced AFD, please see Frequently Asked Questions (updated 20 May, 2008). For help decoding the image legends, please see Legends Help. Previous Version ZCZC SEAGPHAFD ALL TTAA00 KSEA 221754 AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE SEATTLE WA 955 AM PST SUN NOV 22 2009 .SYNOPSIS...A LOW PRESSURE CENTER OVER WASHINGTON (image) WILL SHIFT INLAND AND HIGHER PRESSURE WILL BUILD INTO THE REGION TODAY. THIS WILL GIVE RATHER BLUSTERY WINDS...ESPECIALLY ON THE COAST AND IN THE STRAIT OF JUAN DE FUCA. SNOW SHOWERS WILL BE HEAVY AT TIMES IN THE MOUNTAINS. A WARM FRONT WILL MOVE THROUGH THE AREA MONDAY (image) AND MONDAY NIGHT (image) FOLLOWED BY ANOTHER WARM FRONTAL SYSTEM LATER IN THE WEEK. && .SHORT TERM...ANOTHER BLUSTERY DAY FOR WRN WA...WINDY ON THE COAST AND THRU THE CHEHALIS GAP...AND WESTERLY GALES SHUD DVLP IN THE STRAIT BY NOON. THE WIND ADVISORY SHUD PROBABLY BE EXTENDED NORTH THRU THE METRO AREA AND HOOD CANAL FOR RATHER BLUSTERY WEATHER DVLPG THRU THE MORNING AND PEAKING MIDDAY. WHEN I DO THAT I WILL HAVE THEM ALL EXPIRE AT 3PM. SHOULD BE A GOOD PSCZ WIND PATTERN BY AFTERNOON...SO AFTER THE SURGE OF SW 20-30MPH WINDS THIS MORNING THRU THE SEATTLE METRO AREA THE WIND WILL SETTLE DOWN AS THE PSCZ SETS UP. THE WINTER STORM WARNING FOR THE CASCADES PROBABLY NEEDS A REFRESH AS WELL...IT EXPIRES AT 1PM AND AS THE WESTERLY OROGRAPHIC SNOW KICKS IN BY AFTERNOON SO THAT NEED TO BE EXTENDED INTO THE EARLY EVENING. WINDS IN THE CASCADE PASSES INCREASING TO WESTERLY 20-30 MPH BY AFTERNOON AND GUSTS TO 50 ON THE RIDGETOPS...A LOT LESS THAN THE PEAK WINDS OVERNIGHT: AT THE VERY TOP OF CRYSTAL MOUNTAIN ARND 1AM LAST NITE STRONG SE WINDS OF 50 MPH GUSTED TO ARND 95MPH...QUITE A WINTER STORM UP HIGH (image). THE SYSTEM FOR MONDAY WILL BE WEAKER AND WARMER...ALTHOUGH IT WILL PROBABLY MUSTER SSE GALES JUST AHEAD OF THE FRONT OVER THE COASTAL WATERS MON MORNING....BY MON NITE THE GRADIENT OVER WRN WA IS QUITE LIGHT. HERE IS THE FRONT BEFORE AND AFTER (image) (image) THE 850MB TEMPS STARTS ARND -3C AND POSTFRONTAL IS ARND 0C...SO CALL IT A WARM OCCLUSION. THE WEATHER PATTERN NEXT WEEK LOOKS A LOT LIKE WHAT EL NINO IS SUPPOSED TO BRING...WARMTH AND FAIRLY WEAK SYSTEMS. 19 .LONG TERM...ANOTHER WARM FRONT WILL BRUSH THE NORTHWEST CORNER OF THE AREA AS IT MOVES INTO BRITISH COLUMBIA. THE TRAILING COLD FRONT WILL DRAG SLOWLY ACROSS THE FORECAST AREA WEDNESDAY NIGHT THROUGH THURSDAY FOLLOWED BY ITS UPPER TROUGH THURSDAY NIGHT AND FRIDAY. MCDONNAL && .HYDROLOGY...THE ONLY CURRENT RIVER FLOOD PROBLEM IS THE SKOKOMISH RIVER. IT IS JUST ABOVE FLOOD STAGE THIS MORNING AND WILL LIKELY REMAIN ABOVE FLOOD STAGE INTO MONDAY. THIS IS DUE PRIMARILY TO RELEASES OF WATER FROM CUSHMAN DAM #2. THE SYSTEM MOVING THROUGH WESTERN WASHINGTON TODAY IS NOT A FLOOD-PRODUCING RAIN SYSTEM BUT RATHER A HEAVY MOUNTAIN SNOW SYSTEM. THE FRONTAL SYSTEMS EXPECTED TO ARRIVE DURING THE COMING WEEK WILL BE ACCOMPANIED BY HIGHER SNOW LEVELS -- PROBABLY IN THE 5000 TO 7000 FT RANGE. ONE FRONT IS FORECAST TO MOVE THROUGH MONDAY AND MONDAY NIGHT WITH ANOTHER SYSTEM MOVING SLOWLY THROUGH WEDNESDAY NIGHT THROUGH THURSDAY NIGHT. THEY ARE NOT EXPECTED TO PRODUCE ENOUGH PRECIPITATION TO CREATE FLOODING CONCERNS. NO FLOODING IS EXPECTED AT ANY TIME ON THE GREEN RIVER. && .AVIATION...SURFACE LOW PRES THAT MOVED ONSHORE EARLIER THIS MORNING IS NOW SHIFTING INTO WRN WA ALONG WITH THE UPPER TROUGH (image). ONSHORE FLOW WILL PERSIST TODAY WITH STRONG WLY FLOW ALOFT (image). THE ATMOSPHERE WILL REMAIN MOIST AND SLIGHTLY UNSTABLE THIS MORNING WITH WRAP AROUND MOISTURE KEEPING LOWER CIGS AND RAIN (image) AROUND A LITTLE LONGER THAN EXPECTED. CIGS SHOULD IMPROVE TO VFR AFTER 21Z WITH DECREASING MOISTURE. WITH PARTIAL CLEARING TONIGHT...EXPECT AREAS OF FOG TO FORM...MOSTLY IN FAVORED SHELTERED VALLEYS. A PUGET SOUND CONVERGENCE ZONE /PSCZ/ WILL DEVELOP N OF THE SNOHOMISH/KING COUNTY BORDER LATER THIS MORNING AND THEN MOVE S DURING THE AFTERNOON...BECOMING QUASISTATIONARY OVER THE N CENTRAL PUGET SOUND. AT THIS TIME...IT LOOKS LIKE THE PSCZ WILL REMAIN N OF KBFI. WINDS WILL BACK TONIGHT AS THE PRES GRADIENT BECOMES SLY... THUS EXPECT THE PSCZ TO DRIFT BACK TO THE N AND THEN DISSIPATE LATER TONIGHT. KSEA...CIGS 1-2K FT AND VIS 3-5SM MAY HOLD IN A LITTLE LONG THAN EXPECTED DUE TO WRAP AROUND MOISTURE. STILL EXPECT IMPROVEMENT TO VFR CONDITIONS BY 21Z. SW WIND 10-15 KT W/ GUSTS TO 30 KT TODAY...BECOMING S 3-6 KT THIS EVENING. DTM && .MARINE...STRONG ONSHORE FLOW TODAY BEHIND A LOW PRESSURE SYSTEM WHICH IS NOW SHIFTING INTO ERN WA. GALES ALONG THE COAST SHOULD CONTINUE THROUGH MOST THE DAY BEFORE EASING. THE WINDS IN THE STRAIT TOOK A LITTLE LONGER THAN EXPECTED TO COME AROUND WLY...BUT SHOULD STILL POP UP TO GALE FORCE BY NOON. WINDS HAVE COME UP SLIGHTLY AROUND PUGET SOUND BUT OBS SO FAR ARE FAR SHORT OF GALES...SO I WILL KEEP A SMALL CRAFT IN EFFECT TODAY. DTM && .SEW WATCHES/WARNINGS/ADVISORIES... WA...A FLOOD WARNING REMAINS IN EFFECT FOR THE SKOKOMISH RIVER IN MASON COUNTY. A WINTER STORM WARNING REMAINS EFFECT FOR THE CASCADES TODAY. WIND ADVISORIES ARE IN EFFECT FOR THE STRAIT OF JUAN DE FUCA AND THE ADMIRALTY INLET AREAS...SOUTHWEST INTERIOR...LOWER CHEHALIS VALLEY...SEATTLE TACOMA BREMERTON METRO AREA...HOOD CANAL. PZ...GALE WARNINGS ARE IN EFFECT FOR ALL WATERS EXCEPT PUGET SOUND TODAY. SMALL CRAFT ADVISORY PUGET SOUND TODAY. A SMALL CRAFT ADVISORY REMAINS IN EFFECT FOR ROUGH GRAYS HARBOR BAR CONDITIONS TIL 4 PM MON. $$ WWW.WEATHER.GOV/SEATTLE FOR AN ILLUSTRATED VERSION OF THE FORECAST DISCUSSION...PLEASE SEE WWW.WEATHER.GOV/SEATTLE/GAFD/LATEST_WEBAFD.HTML. |
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