Area Forecast Discussion National Weather Service Seattle WA 923 AM PST Friday Nov 28 2014
Synopsis: — a cold front will move Southeastward across the area this morning. Locally heavy rain is expected ahead of this front. Rain will diminish from the North behind the front this afternoon. A convergence zone will form over Northern portions of the Puget Sound area tonight with snow levels lowering to below 300 feet overnight. A modified arctic cold front will move South through the area late tonight into Saturday morning — with light snow showers expected with the front. Weak high pressure aloft and low level offshore flow will bring mainly cool and dry weather Sunday through early next week.
Short term: — a cold front is moving South across West WA this morning. Wind observations showed the cold front extending from Seattle to a little South of Westport at 8 AM PST/8am this morning. The bad news is that moist Southwest flow aloft is still running up over the top of the front with the rain line extending from the North WA coast across Bellingham — or about 80 miles behind the surface front. The front should reach a Chehalis to North Oregon coast line around 1 PM PST/1pm — and finally clear Lewis County by 4 PM PST/4pm. The frontal rain will probably taper off about 3 hours after frontal passage — finally clearing Lewis County early this evening.
Heavy rain will continue over the mountains today — but will diminish from the North. This will maintain the chance of flooding on a number of rivers flowing off the Cascades — as well as the Skokomish river in Mason County See the hydrology section below for more detail.
W flow through the Strait will increase this afternoon which will allow a convergence zone to form over North Puget Sound behind the front late this afternoon and tonight. This will become a bigger factor late tonight when the air mass cools enough for snow to reach the ground.
The cold upper level trough over Central B. C. Will move steadily South until it is centered over West WA late tonight. A strong surface high will also move South over the Southern B. C. Interior tonight. The upper level trough will cool the air mass enough tonight for snow to develop in the convergence zone over North King County and Snohomish County before midnight. This could result in a couple of inches of snow accumulating.
Strong Fraser river outflow will develop tonight as the surface high settles over South B. C. The cold air spilling through the Fraser river valley will form a modified arctic cold front that will move South across the West WA interior late tonight. This front may drop another inch of snow over the interior late tonight and Saturday morning. Some of the cold air will spill West out the Strait of Juan de Fuca. This could result in 1 to locally 3 inches of snow along the Strait where upslope effects from the Olympics will play a role. Snow levels will fall to near sea level by Saturday morning.
The area most likely to receive the most snow is the North Puget Sound where the arctic front will add to the convergence zone precip. Up to 3 inches is the best guess for this area with around an inch or less across the rest of the interior. However if the convergence zone is more intense and the modified arctic front more vigorous or slower — then a couple more inches could fall. Likewise — if the upslope flow along the Strait is stronger — a couple more inches could fall there. Winter Weather Advisory may be necessary for at least the convergence zone area tonight.
A high wind Watch remains in effect for parts of the North interior tonight and Saturday for the strong Northeast Fraser valley outflow winds. 4 PM PST mesoscale models are still showing spotty 35kt/40mph North Northeast winds late tonight and Saturday morning. If the 4 AM PST models show the same — upgrading to High Wind Warning may be necessary.
Models are in good agreement in ending the precip as the upper level trough axis moves Southeast across the area on Saturday. In fact — precip may end early in the afternoon with clearing from the North. With all the cold air in place — Saturday night will be cold with Lows dropping into the 20s — with upper teens over parts of the North interior. Sunday should be clear and cold with Highs in the lower to middle 30s. Kam
Long-term: — previous Discussion: from the 310 am
Area Forecast Discussion — Northern stream systems dropping Southeast from the Gulf of Alaska — or Southern stream moisture wrapping Northward from off California. Models still cannot come to any agreement over the Pacific Northwest where flow will remain somewhat confluent. The ECMWF — nam12 and Canadian solutions weaken Northern stream systems before they move into Washington during the period. The 10 PM PST GFS is aggressive in bringing Southern stream moisture into Southwest Washington Monday Thursday rain there and more clouds to the North. Forecasts for now will be kept dry with partly cloudy conditions and temperatures will be forecast to slowly moderate toward normal. Albrecht
Hydrology — an obvious band of moderate to heavy rain is over Northwest Washington this morning. The rain is falling mainly over the Olympics and the Cascades of Whatcom — Skagit — and Snohomish counties. The rain is moving slowly South with considerable tapering in the rainfall evident over Northwest Whatcom County in the last few hours. Heavy rain will begin in the Cascades of King County soon and may spread to Pierce and Lewis counties — though the forecast is for less rain in the Southern counties overall.
Discussing the rivers from North to South —
The Nooksack river will touch flood stage on its South fork and may also reach flood stage on the main stem at North Cedarville and possibly ferndale. Rainfall over Whatcom County is tapering and cold air is beginning to move in. Any flooding on the Nooksack river is likely to be minor.
The Skagit river has reached the moderate flood level at Concrete and is forecast to reach the moderate level at Mount Vernon tonight. The us army corps of engineers is operating the dams on the Skagit and Baker rivers. Their plan is to prevent major flooding on the Skagit. Heavy rain will end in a few hours and the uncontrolled reaches of the rivers will stop rising shortly after that.
The stillaguamish river is rising and heavy rain is likely to persist for several more hours in its basin. Both the North fork and the main stem near Arlington seem destined to have minor flooding and a warning may go out soon. Moderate flooding is possible.
The uncontrolled Skykomish river may have the biggest flood of this event. At Gold Bar the river rose above the moderate flood level this morning. Several more hours of heavy rain are likely in the basin. Upstream river gauges are still showing rises. Major flooding is a definite possibility on the Skykomish. This will propagate downstream today and could cause moderate flooding on the reach of the Snohomish river near monroe and also some flooding at Snohomish.
It has only recently begun raining hard in the Snoqualmie river basin and both the Snoqualmie and Tolt rivers have shown only minor rises so far. However they both started out running high and minor flooding is forecast on both rivers tonight.
No flooding is forecast on any rivers South of the Snoqualmie. When the front passes over the area this afternoon and evening there could be some heavy rain and rivers will rise somewhat. A Flood Watch remains in effect for all counties from Skagit to Lewis.
In the Olympics the Skokomish river will remain above flood stage today through Saturday. The Bogachiel river in the far Northwest Olympic peninsula appears to have crested a couple of feet below flood stage. Both the Elwha and Dungeness rivers also appear to be cresting below flood stage.
Cold and dry weather will follow the front tonight. Rivers will recede and the threat of flooding will end for the foreseeable future. Burke
Aviation: — a cold front is over Western WA this morning moving slowly South. Westerly flow aloft. Areas of fairly low ceilings and reduced visibility in the heavier rain. This front will move Southeast out of the area by this evening and cold high pressure over interior B. C. Will give Northeast Fraser river outflow winds to the North interior tonight. With colder air tonight and a cold upper trough moving through the area tonight there will likely be some areas of snow later tonight — especially the Strait of Juan de Fuca and in any convergence zones. The upper trough will exit later Saturday afternoon and the dry cold air will settle over the area.
SeaTac — a cold front is over Seattle this morning and fairly heavy rain and the wind shifting from Southwest to Northerly here at the office just after 8am — the wind shift to Northerly should be through sea tac by 9am. The wind may turn back to Southerly this evening — although variable is also a good forecast — and then Northerly winds will increase Saturday as the cold air drops over the area. As the cold air out of B. C. Reaches Seattle overnight showers should turn over to snow showers and an inch is not a bad bet at sea tac late tonight with Everett a better bet for snow up in a the Puget Sound convergence zone/modified arctic front. The radar will give the best forecast tonight as that sets up.
Marine — a strong cold front will shift South through Western WA this morning and Small Craft Advisories are in effect for all waters. Winds are turning to Northerly over Puget Sound. I am not so sure that the wind will go back to Southerly in Puget Sound late this afternoon — a the Puget Sound convergence zone pattern is a good bet. Tonight there will be Fraser river outflow Northeast Gales for the Northern inland waters which will probably spread South through the Eastern half of the Strait overnight. The outflow will ease late Saturday — but offshore flow will continue sun and Monday with high pressure East of the Cascades.
Puget Sound — Small Craft Advisory all waters today.
Gale Warning Northern inland waters tonight and Saturday. Small Craft Advisory Grays Harbor bar for rough 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