Area Forecast Discussion National Weather Service Seattle WA 3:35 PM PDT Wednesday Jul 29 2015
Synopsis: — an upper ridge will give dry and hot conditions to the interior of Western Washington through the weekend. An upper trough will bring marine air and cooling to the area starting Sunday night through the middle of next week.
Short term: — the center of a strong and warm upper level ridge over the South Central united states is expanding as it retrogrades toward the four corners region of the desert Southwest through Friday. Flow aloft is light onshore while low level flow is light North to Northwest. A thermal trough at the lower levels is developing from the Olympics Southward through the South interior of Western Washington then into the willamette valley of West Central Oregon.
As the surface pattern develops — 850 millibars temperatures warm to around 21c Thursday through Friday afternoon. With the axis of the thermal trough inland — expect the coast away from the beaches to get warm — but not hot. Areas near the water North of Seattle and in the North interior will be cooled by the 10 to 15 knots North to Northwest breezes keeping Highs in the lower to maybe middle 80s. The Seattle metropolitan area Southward in the interior valleys will get quite hot with widespread Highs in the lower to middle 90s. It will also get hot a few miles East of interstate 5 in the Northern interior from Lynden down to Arlington and Snohomish. Nighttime Lows will fall into the 50s in most rural areas but will stay in the lower 60s in the heat island of the Seattle Tacoma metropolitan areas. It appears that Friday will be the hottest day in the Seattle area with SeaTac airport possibly reaching 94 degrees. Heights and upper level temperatures cool slightly on Saturday with little change in the surface pattern. At this point will go for a 90 degree maximum at SeaTac on Saturday.
Afternoon dewpoint temperatures in the Central Puget Sound air will drop into the upper 40s during the time of maximum heating with the mixing of dry air aloft downward toward the surface with afternoon breezes. So conditions are not expected to get overly oppressive outdoors. But with the hot maximum temperatures and warm overnight Lows — will issue Heat Advisory from midday Thursday until midnight Saturday night for the Seattle — Tacoma — East Puget Sound lowlands — and Southwest interior zones. Albrecht
Long-term: — while the four corners high remains in place through the middle of next week — upper level heights over the Pacific Northwest start a downward trend later Saturday into Sunday for more cooling and possibly a partial inland push of marine air. A full fledged and strong marine push appears to be likely for Sunday night into Tuesday as an upper low moving toward the Alaskan panhandle drops Southeast toward Southern British Columbia and Western Washington midweek. The movement of this upper low appears to be propelled by the development of a weak block along 150W that is being aided by the development of a vertically stacked low near cold bay Alaskan. The 5 AM PDT solutions are in increasingly good agreement and ensemble members are more tightly clustered around the operational GFS solution today. Confidence is still not incredibly high — but the extended forecast will be moved toward this cooler — cloudier — and by Wednesday and Thursday potentially wetter — solution. Albrecht
Climate — on this date back in 2009 the warmest day on record in Seattle occurred with a high of 103 degrees. In addition to being the warmest day on record July 29th is also the driest day on record at Seattle Tacoma airport. Since records started in 1945 there has been a total of just 0.31 inches of rain recorded on this day. Measurable rain has occurred only 4 times ( 0.17 inches 1993, 0.11 inches 2008, 0.02 inches 1955 and 0.01 inches in 1981 ). Felton
Aviation: — an upper ridge will remain over the region. Contd Westerly flow aloft. Low level Northwest flow will become ny late tonight. Visual Flight Rules conditions are expected across much of the area. There will likely be patchy lIFR ceilings/vsbys on the coast after 0600 utc.
SeaTac — Visual Flight Rules. Winds will remain Northerly.
Marine — high pressure offshore with lower pressure East of the Cascades will result in onshore or Northwest flow through Friday. The flow will become briefly Northerly or offshore early Thursday and Friday mornings due to a thermally induced area of low pressure trying to set up along the Washington coast.
Onshore flow will strengthen this weekend — and this will result in the possibility of Gale force winds over parts of the Strait of Juan de Fuca on Sunday. The flow will strengthen further on Monday — leading to an increased threat of Gale force winds over parts of the Strait of Juan de Fuca.
Fire weather:: — a very dry air mass aloft is settling over Western Washington through Friday night. 850-700 millibars layer average relative humidity will be running as low 10 percent between now and then. Mid level haines Index will be solid 5 with values reaching 6 at times over the Central Cascades — indicating a dry and unstable middle level air mass. Will easily reach criteria for the dry and unstable category of Red Flag Warnings through Friday evening. As a result — Red Flag Warning will soon be in effect. It will be targeted to locations above 2000 feet elevation — which will stick up above the inhibiting effect of low level subsidence inversions. With such a dry air mass — leftover fuel moisture from recent rains will quickly become a memory. Over the mountain zones — this kind of air mass supports active fire behavior and rapid growth on existing fires. Haner
Puget Sound — small craft remains in advisory until 11 PM PDT tonight for the coastal waters from James Island to cape shoalwater out to 10 nm.
An illustrated version of this forecast Discussion: can be seen at http://www.weather.gov/seattle/gafd/latest_webafd.html
A complete list of airport weather stations used for the METAR reporting system is at: http://adds.aviationweather.gov/metars/stations.txt . Regional airports weather station designations are at the bottom of this page.
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Acronyms ACARS Aeronautical Radio Communications Addressing Reporting System VFR Visual Flight Rules Numerical Forecast Models Abbreviations CMC Canadian Meteorological Centre ECMWF European Centre for Medium-Range Weather Forecasts GEM Global Environment Multiscale (CMC's short-range model) GFDL Geophysical Fluid Dynamics Laboratory's Hurricane Model GFS Global Forecast System (formerly known as the AVN and MRF) MM5 Penn State University/National Center for Atmospheric Research's Mesoscale Model NAM North American Mesoscale (formerly known as the Eta) NGM Nested Grid Model NOGAPS Navy Operational Global Atmospheric Prediction System RUC Rapid Update Cycle Forecast Model SREF Short Range Ensemble Forecasts UKMET United Kingdom Meteorological Office Washington Airports and other NWS stations elevations in meters N W elev WA ARLINGTON MUNI KAWO 48 10N 122 10W 42 WA BELLINGHAM KBLI 48 48N 122 32W 50 WA WHATCOM (BLI) KHUH 48 57N 122 35W 26 WA BREMERTON NTNL KPWT 47 30N 122 45W 147 WA BURLINGTON/MT V KBVS 48 28N 122 25W 43 WA CHEHALIS CENTRAL KCLS 46 41N 122 59W 54 WA COLVILLE MUNICIP KCQV 48 32N 117 52W 572 WA DEER PARK KDEW 47 58N 117 26W 668 WA EASTSOUND/ORCAS KORS 48 42N 122 54W 8 WA ELLENSBURG KELN 47 02N 120 32W 519 WA EPHRATA KEPH 47 18N 119 31W 383 WA EVERETT KPAE 47 55N 122 17W 180 WA FAIRCHILD AFB KSKA 47 37N 117 39W 750 WA FORT LEWIS/GRAY KGRF 47 04N 122 34W 92 WA FRIDAY HARBOR KFHR 48 31N 123 02W 32 WA HANFORD KHMS 46 34N 119 35W 223 WA HOQUIAM KHQM 46 58N 123 56W 7 WA KELSO LONGVEIW KKLS 46 07N 122 54W 5 WA MOSES LAKE KMWH 47 12N 119 19W 362 WA OAK HARBOR AIRPA K76S 48 15N 122 40W 58 WA OLYMPIA KOLM 46 58N 122 54W 58 WA OMAK KOMK 48 28N 119 31W 395 WA PASCO KPSC 46 16N 119 07W 121 WA PORT ANGELES KCLM 48 07N 123 30W 85 WA PULLMAN/MOSCOW KPUW 46 45N 117 07W 773 WA PUYALLOP/THUN KPLU 47 06N 122 17W 164 WA QUILLAYUTE KUIL 47 56N 124 33W 54 WA RENTON KRNT 47 30N 122 13W 21 WA SEATTLE/BOEING KBFI 47 33N 122 19W 4 WA SEATTLE/METRO KSEA 47 27N 122 19W 136 WA SEATTLE/WFO KSEW 47 27N 122 18W 130 WA SEATTLE/ARTCC KZSE 47 17N 122 11W 99 WA SEATTLE/CAMANO I KATX 48 12N 122 30W 151 WA SHELTON KSHN 47 14N 123 08W 82 WA SPOKANE KOTX 47 41N 117 38W 727 WA SPOKANE/FELTS KSFF 47 41N 117 19W 609 WA SPOKANE/METRO KGEG 47 37N 117 32W 735 WA STAMPEDE PASS KSMP 47 17N 121 20W 1208 WA TACOMA KTIW 47 16N 122 35W 89 WA TACOMA/MC CHORD KTCM 47 07N 122 28W 98 WA TATOOSH/VOR KTOU 48 18N 124 38W 520 WA TOLEDO WINLOCK M KTDO 46 28N 122 47W 113 WA VANCOUVER KVUO 45 37N 122 39W 8 WA WALLA WALLA KALW 46 06N 118 17W 363 WA WENATCHEE KEAT 47 24N 120 12W 377 WA WHIDBEY IS. NAS KNUW 48 21N 122 39W 14 WA YAKIMA KYKM 46 34N 120 32W 324 WA BURLINGTON/MT V K75S 48 28N 122 25W 43 WA COLVILLE K63S 48 32N 117 52W 571 WA DEER PARK K07S 47 58N 117 26W 668 WA DESTRUCTION ISL 47 40N 124 31W 21 WA PORT ANGELES CG KNOW 48 08N 123 25W 4 WA SMITH ISLAND 48 19N 122 10W 15 WA PEARSON/VANCOUVR K60S 45 37N 122 39W 8 WA WEST POINT (LS) 47 40N 122 34W 3 BC ABBOTSFORD AIRPO CYXX 49 01N 122 22W 58 BC AGASSIZ (AUTO) CWZA 49 15N 121 46W 15 BC ENTRANCE IL AUTO CWEL 49 13N 123 47W 5 BC ESQUIMALT METOC CWPF 48 25N 123 25W 12 BC HOPE AIRPORT (MA CYHE 49 22N 121 28W 39 BC HOPE SLIDE CWKV 49 16N 121 13W 674 BC KELP REEFS CWZO 48 33N 123 14W 1 BC MALAHAT (AUTOB) CWKH 48 34N 123 34W 366 BC MERRY ISLAND CWMR 49 28N 123 55W 8 BC MT SICKER RADAR CXSI 48 51N 123 45W 1 BC NANAIMO AIRPORT CYCD 49 02N 123 52W 28 BC P. MEADOWS CS AU CWMM 49 12N 122 40W 5 BC RACE ROCKS AUTOB CWQK 48 17N 123 31W 5 BC SAND headS (LS) CWVF 49 06N 123 18W 1 BC SATURNA ISL (MAP CWEZ 48 46N 123 02W 7 BC SHERINGHAM AUTOB CWSP 48 22N 123 55W 21 BC SQUAMISH (AUTOB) CWSK 49 46N 123 10W 60 BC VANCOUVER (AUTOB CWHC 49 17N 123 07W 2 BC VANCOUVER INTL A CYVR 49 10N 123 10W 2 BC VIC. HARTLAND AU CWVV 48 31N 123 28W 49 BC VICTORIA (AUTOB) CWLM 48 25N 123 19W 70 BC VICTORIA HARBOUR CYWH 48 25N 123 23W 10 BC VICTORIA INTL AR CYYJ 48 38N 123 25W 19 BC VICTORIA UNIV CWYJ 48 28N 123 18W 39 BC W VANCOUVER AUTO CWWA 49 21N 123 10W 178 BC WHITE ROCK AUTOB CWWK 49 01N 122 46W 15