Area Forecast Discussion National Weather Service Seattle WA 915 PM PST Saturday Nov 22 2014 — corrected headlines
Synopsis: — there will be a lull in lower elevation rain showers and mountain snow much of tonight. A vigorous front will arrive early Sunday morning — renewing rain and mountain snow across Western Washington. The next warm frontal will reach the area Monday night and Tuesday. More weather systems might arrive late next week.
Short term: — radar is picking up batches of showers moving through Western WA this evening under a weakly unstable air mass. There was one lone lightning strike near the West entrance to the Strait earlier and if anything else does occur it would be isolated and confined to near the coast. Otherwise — a relative lull in precipitation is expected at least through the first half of tonight.
Warm advection already working into Western WA will become more noticeable after midnight tonight in advance of the next approaching system. The 6 PM PST high res rapid refresh (hrrr) is indicating some milder air intruding at 850 millibars — especially after midnight as warm advection moisture also increases. Pass temperatures at Snoqualmie and Stevens are already near freezing with a couple degrees warming at 850 millibars expected later tonight below 3500 feet as steadier precipitation with the next front arrives. The question becomes how much snow will fall at the lower passes compared to higher spots like Stevens — paradise — and mount Baker. Models trends suggest a mix or change to rain is possible Sunday morning at Snoqualmie before going back over to snow in the afternoon. Above 3500 feet — precipitation should fall as mostly snow through the day. Latest model runs show steady rain/mountain snow picking up after 3-4 am then tapering to showers Sunday afternoon — except with a period of significant snow in spots favored in stronger Westerly flow such as the passes. Snow levels will also be falling to 3000 feet by that time affecting even Snoqualmie Pass Will go ahead and issue a Winter Storm Warning for the mountains and indicate the heavier amounts over 1 foot mainly above 3500 feet — with locally higher amounts over 1 foot at the ski resorts. Snoqualmie should see more rain than snow but a burst of heavy snow post frontal should give at least a quick few inches. Also — warning amounts seem likely at paradise and alpental which justifies the warning for those zone. Will word the warning statement accordingly.
Showers will decrease rather quickly by Sunday evening as high pressure offshore amplifies and moves over the Pacific Northwest through Monday morning. The ridge will flatten by Monday afternoon allowing warm advection rain under moderately strong Westerly flow. This stalled flow pattern could produce locally heavy rain across Western WA through Tuesday with relatively high snow levels. See the hydro Discussion: below. Mercer
Long-term: — no update from previous Discussion: — heavy rain is likely in the
North Cascades Tuesday and Tuesday night — and perhaps into Wednesday morning. The rest of Western WA should receive light rain while areas in the lee of the Olympics could be fairly dry and blustery with winds aloft mixing down. The middle part of the week looks a little different in each model and the weather forecast for Thursday is especially hard to figure out. The 10 PM PST GFS had rather cool air over Eastern WA with strong offshore gradients across the state and a breezy dry day for Western WA on thanksgiving. Now the models look like some sort of stationary front could be hung up over the region which then merges with the first of a series of new systems tracking along in the Pacific Westerlies.
Hydrology — the 4 AM PST and 10 AM PST GFS runs showed heavy rain in the
North Cascades next week — especially in a 36hr period through midday Wednesday. The 4 AM PST ECMWF spread the rain out over the entire Cascade range — had much less rain — and shifted the baroclinic zone back North into B. C. Rather than letting it peter out over the Cascades like the 4 AM PST GFS. A hydro outlook probably could be confined to just Whatcom and Skagit counties based on the GFS while the Euro probably does not have enough precip for rivers to make flood stage.
Aviation: — moderate Westerly flow aloft. The air mass is moist and somewhat unstable. A cold front well offshore will rapidly sweep across the area Sunday morning and will be followed by post frontal onshore flow Sunday afternoon and evening.
Shower activity is generally confined to the West slopes of the Olympics and Cascades this evening. Shower activity will become more widespread late tonight into Sunday morning as the offshore frontal system moves across the area 7 AM PST 9 AM PST. Showers and clouds will become increasingly tied to the terrain and a Puget Sound convergence zone midday Sunday behind the front. Ceilings will generally range from 010-025 through midday Sunday. Visibilities will generally remain above 5 miles — with local 2-4 statute mile rain mist with the front and in heavier post frontal showers on Sunday. Albrecht
SeaTac — the terminal is generally in an Olympic rain shadow this evening. Expect showers to increase about 4 AM PST as the offshore front approaches the area then taper to occasional rain showers about 10 AM PST sun behind the front. South wind 6-9 knots this evening will increase to 14-18 knots with g28 knots 15-9 AM PST then turn Southwesterly 9-12kt 12 PM PST sun. Wind West Southwest 40 knots at 5000 feet ahead of the front and Westerly 45 knots behind the front. Albrecht
Marine — winds have generally eased this evening. Expect Small Craft Advisory South to Southwest winds to ramp up over all of the waters after midnight as a frontal system now nearing 130w approaches the waters. The front will move onto the coast about 7 AM PST sun and through the inland waters by about 9 AM PST sun. Expect Gale force Westerly flow through the Central and Eastern Strait of Juan de Fuca behind the front through early Sunday evening with Small Craft Advisory conditions in the other waters gradually relaxing Sunday afternoon.
Flow will shift to SouthEasterly late Sunday night into Monday as a warm front moves Northeast across the waters. The front will become stationary just North of the waters Tuesday through Wednesday with strong South Southwest flow over most of the waters. Albrecht
Puget Sound —
Gale Warning Central and Eastern Strait. Small Craft Advisory remaining waters. Small Craft Advisory for rough Grays Harbor bar conditions.
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