Area Forecast Discussion National Weather Service Seattle WA 9:00 PM PDT Thursday May 28 2015
Synopsis: — the Pacific Northwest will remain between a weak upper low to the North over Central British Columbia and an upper ridge to the South. Westerly flow aloft will prevail with generally dry conditions and above normal high temperatures through the weekend. It will be unstable over the mountains with a threat of afternoon and evening thunderstorms Friday. Another upper level low will move into the Pacific Northwest Sunday night and Monday bringing showers and the potential for thunderstorms to much of the forecast area. High pressure will return by middle week.
Short term: — upper level low pressure will shift Southward tonight over Central B. C. With light Westerly flow aloft and stronger low level onshore flow. The air mass remains unstable over the Cascades and Olympics early this evening with the most active area of thunderstorms over the Cascades of Pierce and Lewis counties. While locally moderate to heavy rain is likely occuring with these storms — cells should diminish over the next hour with loss of diurnal heating. The GFS40 forecast sub zero li`s actually encompass areas further Westward Friday afternoon. It could be somewhat active over the Olympics and Cascades — but pushing off to the East early Friday evening as Westerly flow increases. The forecast will be updated to account for mountain thunderstorms.
It was a warm day across parts of interior Western WA with Highs in the low 80s around much of Puget Sound and the Southwest interior. Light Westerly flow kept temperatures cool along the coast — Strait — and around the entrance to the Strait. Onshore gradients are somewhat stronger this evening which should continue into Friday. This should cut down high temperatures a bit across the interior — but less so around the Central Sound Eastward toward the foothills where the warmest Highs near 80 will likely occur. The hrrr shows stratus pushing inland reaching Shelton but not expanding much further inland due in part to increasing cirrus. The last visible picture of the day backs up this idea with stratus not yet pushing through the Chehalis gap.
The ridge builds a bit stronger Northward over the region on Saturday. Onshore flow strengthens and the increased marine influence should knock high temperatures down a few more degrees but still in the 70s over much of the interior. It should be mostly sunny across the interior by afternoon but staying cloudy along the coast.
A transition to unsettled weather will begin on Sunday. An upper low off the Northern CA coast will eject Northeast clipping Western WA. Models differ on timing and details but showers will probably develop across the area during the Sunday through Monday period. The flow aloft will become diffluent ahead of the low on Sunday with the potential for elevated convection above the marine layer over Western WA lowlands. If the upper low comes in negative tilt like the GFS40 shows Sunday night and Monday — dynamic forcing could be sufficient for thunderstorms across Western WA including the lowlands. There is still a lot of uncertainty as to the timing and track of the low so no changes will be Made at this point. Mercer
Long-term: — no changes from previous Discussion: — medium range models continue to highlight building heights over the region on Sunday Sunday ahead of a negative tilted upper level trough approaching from the Southwest. This will likely boost high temperatures Sunday afternoon as well as help pull moisture up from the South and bring an increasing threat of showers late Sunday afternoon and evening over the South — spreading North through the night. The flow pattern is a little more conducive to thunderstorms especially as the upper low will moves inland Monday. Showers and clouds will linger over the area Tuesday and Tuesday night with Highs in the 60s. And Lows in the middle to lower 50s. Weak high pressure aloft will return to the region by midweek bringing drier conditions and a typical pattern of morning clouds and afternoon sunshine.
Aviation: — the air mass is mostly dry and somewhat stable with light Westerly flow aloft. The air mass is more unstable over the Cascades — but early evening thunderstorms will dissipate quickly this evening with loss of daytime heating.
Marine stratus with patchy fog is sitting over the waters of the Pacific. The stratus will push Eastward into the Strait of Juan de Fuca overnight and through the Chehalis Gap to around Shelton and Olympia. The hrrr and GFS show less extensive stratus and fog inland as compared to this morning as cirrus coming into the area from the Southwest inhibits radiational cooling to some degree. Will keep fog and stratus out of the Bellingham — sea — and Seattle/Boeing Field terminals through Thursday morning but continue to show fog in vicinity or tempo stratus at Olympia and pae. Stratus that does occur will be low based and shallow with bases 004-006 msl. Tops will be generally remain below 1000 ft. Visibility restrictions in fog (under 5/8 mile visibility) will occur where the stratus intersects the ground. Stratus will all burn back to the Pacific coastline by 11 AM PDT.
The air mass will be slightly more unstable aloft across the area on Friday than today with lifted indicates -1 to -3 during the late afternoon and evening hours. Moisture and organized lift is lacking — so the threat for thunderstorms will be limited to the Cascade crest area 2 PM PDT 9 PM PDT with only a 10 percent chance that a buildup over the Olympics will give an isolated thunderstorm there. The lowlands are expected to be dry Friday aftn. Albrecht
SeaTac — scattered broken cirrus with North wind 8-10 knots easing to Northeast 5 knots after 12 AM PDT then becoming light and variable in the morning. A Northwest wind to 10 knots will resume after 3 AM PDT. Expect stratus to remain West of the terminal area Friday morning. Albrecht
Marine — high pressure centered offshore with lower pressure inland will give light to moderate onshore flow through the weekend. Small craft advisory conditions can be expected during the late afternoon and overnight hours in the Central and Eastern portions of the Strait of Juan de Fuca.
Areas of fog over the coastal waters will push in through the Strait of Juan de Fuca overnight. Morning fog will be quite patchy in the inland waters during the morning hours on Friday. Albrecht
Puget Sound — Small Craft Advisory Central and Eastern Strait of Juan de Fuca through 6 AM PDT Friday.
You can see an illustrated version of this forecast Discussion: 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