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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 041034 AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE SEATTLE WA 300 AM PDT SAT JUL 4 2009 .SYNOPSIS...SUNNY AND WARM WEATHER WILL CONTINUE THROUGH THE HOLIDAY WEEKEND...WITH AREAS OF MORNING LOW CLOUDS AND FOG CONFINED MAINLY TO THE COAST. AN UPPER TROUGH WILL MOVE INTO THE REGION SUNDAY NIGHT AND MONDAY FOR A TRANSITION TO COOLER WEATHER WITH A CHANCE OF SHOWERS FOR NEXT NEXT WEEK. && .SHORT TERM...AN UPPER LEVEL RIDGE REMAINS OVER THE PACIFIC NORTHWEST WITH 500 MB HEIGHTS OVER WESTERN WASHINGTON AROUND 5800 METERS (image). THERE HAVE BEEN SOME CHANGES AT LOW LEVELS THAT WILL MAKE THE FORECAST FOR THE NEXT COUPLE DAYS SLIGHTLY TRICKY. THE SURFACE GRADIENTS TURNED WEAKLY ONSHORE FRIDAY EVENING AND A SHALLOW LAYER OF COASTAL STRATUS HAS BEEN MOVING INLAND SLOWLY OVERNIGHT... MAINLY THROUGH THE CHEHALIS GAP AND TO A LESSER EXTENT DOWN THE STRAIT OF JUAN DE FUCA (image). IN THE INTERIOR THE STRATUS WILL PROBABLY REMAIN LIMITED TO THE SOUTHWEST PART OF THE PUGET SOUND REGION -- IE SHELTON AND PERHAPS BREMERTON -- WITH ONLY PATCHY COVERAGE IN THE STRAIT AND NORTH INTERIOR. IT SHOULD BURN OFF RELATIVELY QUICKLY. BUT MORE IMPORTANTLY THE INFLUX OF SHALLOW MARINE AIR SHOULD HAVE AT LEAST A MINOR EFFECT ON HIGH TEMPERATURES ACROSS THE INTERIOR LOWLANDS. IN GENERAL THE MODELS HAVE UNDERESTIMATED THE STRENGTH OF ONSHORE GRADIENTS. THE BOTTOM LINE IS THAT WE ARE LIKELY TO BE NOT QUITE AS WARM AS FRIDAY ACROSS THE INTERIOR LOWLANDS. THE UPPER RIDGE AXIS WILL MOVE EAST TOWARD THE NORTHERN ROCKIES TONIGHT (image)...AND THE UPPER TROUGH CURRENTLY JUST OUTSIDE 140W WILL APPROACH THE PACIFIC NORTHWEST COAST SUNDAY (image). LOW LEVEL ONSHORE FLOW WILL PROBABLY REMAIN ONLY WEAK...BUT THE 500 MB HEIGHTS WILL FALL TO AROUND 5750 METERS AS COOLER AIR MOVES IN ALOFT. TEMPERATURES OVER THE LOWLANDS WILL BE SIMILAR OR ONLY SLIGHTLY COOLER ON SUNDAY...BUT THE COOLER AIR ALOFT WITH THE TROUGH WILL DESTABILIZE THE AIR MASS FOR A LOW CHANCE OF SHOWERS IN THE CASCADES. IT SHOULD BE MENTIONED THAT THE GUIDANCE IS NOT UNANIMOUS WITH THE IDEA OF SLIGHT COOLING EACH DAY THIS WEEKEND. IN PARTICULAR THE NAM MOS IS FORECASTING TEMPERATURES OVER THE INTERIOR TO BE ABOUT THE SAME AS FRIDAY FOR BOTH DAYS THIS WEEKEND. THE MODELS REMAIN IN GENERAL AGREEMENT THAT THE UPPER TROUGH WILL REACH THE AREA SUNDAY NIGHT (image). UPPER LEVEL HEIGHTS WILL CRASH FURTHER...RESULTING IN A SUFFICIENT INCREASE IN ONSHORE FLOW FOR A MODERATE TO STRONG ONSHORE PUSH. BY MONDAY THE PATTERN CHANGE WILL BE MORE OR LESS COMPLETE -- AN UPPER TROUGH WILL BE SETTLING OVER THE REGION AND LOW LEVEL FLOW WILL BE ONSHORE FOR COOLER WEATHER AND A CHANCE OF SHOWERS (image). MCDONNAL .LONG TERM...THE LONGER RANGE MODELS AGREE THAT THE UPPER TROUGH WILL BE PARKED OVER THE REGION FOR THE EXTENDED FORECAST PERIOD TUESDAY THOUGH FRIDAY. SEE (image) AND (image). THE CURRENT BROAD BRUSH FORECAST LOOKS GOOD. THERE WILL BE MORE CLOUDS THAN SUN ALONG WITH A CHANCE OF SHOWERS. ONSHORE FLOW WILL CONTINUE WITH A FAIRLY STRONG MARINE INFLUENCE... AND TEMPERATURES WILL BE A LITTLE BELOW NORMAL. MCDONNAL && .AVIATION...AN UPPER RIDGE WILL REMAIN OVER THE REGION THRU TONIGHT. INCREASING SW FLOW ALOFT (image). LIGHT ONSHORE FLOW TODAY WILL BECOME FLAT OR WEAK OFFSHORE OVERNIGHT. STRATUS/FOG NOW BLANKETED THE COAST...AND HAS MADE IT AS FAR E AS THE FAR S SOUND (image). THE DEPTH OF THE MARINE LAYER WAS FAIRLY SHALLOW OR BETWEEN 1K AND 1500 FT DEEP. EXPECT THE LOW CLOUDS/FOG TO DISSIPATE OR RETREAT OFFSHORE BETWEEN 18Z AND 20Z...SOONER OVER THE INTERIOR. .KSEA...CONTINUED VFR CONDITIONS. LOW CLOUDS/FOG SHOULD REMAIN WELL S AND W OF THE TERMINAL. EXPECT LIGHT AND VARIABLE WINDS...BECMG NW IN THE AFTERNOON. && .MARINE... LIGHT ONSHORE FLOW WILL PREVAIL OVER THE AREA TODAY. THE FLOW WILL BECOME WEAK OFFSHORE OVERNIGHT BUT THIS WILL BE BRIEF. EXPECT LIGHT ONSHORE FLOW TO RETURN SUNDAY AFTERNOON...WITH A STRONG MARINE PUSH OCCURING SUNDAY NIGHT (image). IT IS POSSIBLE THAT THERE WILL BE A BRIEF PERIOD OF WIND GUSTS NEAR 35 KNOTS WITH THE PUSH AS IT SURGES NORTH ACROSS THE PUGET SOUND...ADMIRALTY INLET...AND THE NORTHERN INLAND WATERS. ANTICIPATE MODERATE ONSHORE FLOW ACROSS THE AREA ON MON. && .SEW WATCHES/WARNINGS/ADVISORIES... WA...RED FLAG WARNING IN EFFECT FOR THE ENTIRE CWA UNTIL MIDNIGHT SUNDAY NIGHT. PZ...SMALL CRAFT ADVISORY FOR WINDS IN EFFECT FOR THE CENTRAL AND EASTERN STRAIT OF JUAN DE FUCA FROM 3 PM THIS AFTERNOON UNTIL MIDNIGHT TONIGHT. $$ WWW.WEATHER.GOV/SEATTLE FOR AN ILLUSTRATED VERSION OF THE FORECAST DISCUSSION PLEASE SEE WWW.WEATHER.GOV/SEATTLE/GAFD/LATEST_WEBAFD.HTML (ALL LOWERCASE)

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