"Translated" Seattle area forecast discussion.

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fxus66 Seattle 230516 AFDSEW

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



— WA —  Winter Storm Warning in effect from 3 AM PST Sunday morning through Sunday afternoon for the Cascades above 3000 feet.

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

Other regional discussions translated:  Seattle | Spokane | Portland | Pendelton | Medford
The Area Forecast Discussion is issued four times daily around 3AM, 9AM, 3PM, and 9PM.
The original form of the discussion, in all CAPS can be found at http://www.srh.noaa.gov/data/SEW/AFDSEW or With hyperlinks to Jargon http://www.wrh.noaa.gov/total_forecast/getprod.php?prod=XXXAFDSEW&wfo=sew&dict=no&version=0
Another form with hyperlinks to images referenced in the discussion can be found at: http://www.wrh.noaa.gov/sew/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.

For a Glossary of jargon used in these discussions see this link

NWS Glossary lookup. . You can either type in the word you are looking for in the box below or browse by letter.


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To learn about cloud types see this article with pictures

Discussion Heights*:

Under normal conditions, a 1000 mb height is near the surface (sea level - 400 ft),
standard sea level pressure is 1013.25 mb at 15C°
By looking up current pressure and temperature, we can calculate what the standard mb height levels are today.

Typical Heights* values above the 850mb level.:

Standard sea level pressure is 1013.25 mb at 15C° — also known as a standard atmosphere.
Most of the time,
1000 mb is near the surface (sea level - 400 ft) (In really deep low pressure, Sea level pressure can drop well below 990mb.)
850 mb is near 1,500 meters (5,000 ft),
700 mb is near 3,000 meters (10,000 ft),
500 mb is near 5,500 meters (18,000 ft),
300 mb is near 9,300 meters (30,000 ft).
*Note: When 500mb heights are said to be "rising to 540" it means 5400m above sea level. (height above zero geopotential meters.)
But, confusingly, when discussing 850mb heights, the convention is to omit the leading 1, so that 850mb heights of "540 dam" are actually 1540 meters above sea level.

(What committee came up with all this? I guess we have to remember that bits used to be very expensive.)
Just remember that when (constant pressure) heights rise it means that the pressure on the surface is rising too. More on heights and how they are reported: http://www.theweatherprediction.com/basic/obs/

For Cloud levels (aka ceilings) in the untranslated discussion: Just add two zeros to the right to get the actual forecast cloud level in feet. So 030 is 3,000ft.

Alternate Weather Jargon glossaries:
NWS official glossary -- Very complete except for idiosyncratic forecasters Or use above form.
Contractions only from NOAA
METAR interpretation http://weather.cod.edu/notes/metar.html
NOAA's official abbreviations http://weather.cod.edu/notes/abrv.html

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