"Translated" Seattle area forecast discussion.

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fxus66 Seattle 191143 AFDSEW


Area Forecast Discussion National Weather Service Seattle WA 344 AM PST Friday Dec 19 2014


Synopsis: — an upper level trough will bring showers to the coast and scattered showers inland today. A pair of strong warm fronts will bring rain and wind to Western Washington tonight through this weekend. An upper level ridge will move briefly over the region Monday for drier weather. Additional frontal systems will reach the region next week.



Short term: — Infrared satellite imagery and radar shows last evenings cold front has moved East of the area. The upper level trough just offshore will move inland over West WA late this morning then weaken rapidly as it moves East of the Cascades this afternoon. The air mass will be unstable today with numerous showers along the coast and scattered showers inland. GFS shows lifted indices as low as -2 over the coastal waters today — but with very little lightning activity so far I will leave out the mention of thunderstorms.

A strong zonal jetstream has formed over the Northeast Pacific and will drive a couple of strong wet warm fronts across West WA tonight through this weekend. Model solutions are in fairly good agreement through Saturday — but continue to evolve regarding some details and especially with regard to precip amounts.

There is good consistency and agreement that light rain will develop this evening as the first strong warm front approaches the region. GFS/ECMWF bring this first warm front to the coast early Saturday morning. Strong South Southeast winds will develop along the coast and over the North interior late tonight ahead of the warm front. A high wind Watch will be issued for these areas.

A new wrinkle is that the models resolve a second warm front trailing right behind the first. The second warm front should reach the WA coast late Saturday afternoon — with an associated weak surface low moving across Vancouver Island early Saturday evening. It now looks like there will be a brief lull in the strong South Southeast winds between the two warm fronts — but there will be a surge of West Southwest wind behind the surface low Saturday evening. The high wind Watch runs through Saturday evening to cover the second warm front and the trailing surface low. Models have been pretty consistent showing up to high end Wind Advisory winds for the coast/N interior through this period — but there is still some uncertainty regarding the strength of each of these features. Some spotty high winds are within reach — hence the high wind watch.

Periods of heavy rain are still expected over the mountains — but resolving these features into two warm fronts with a surface low has had an impact on the middle level wind field which will generate the upslope flow over the mountains. Strong South flow develops tonight ahead of the first warm front — but rather than shifting directly to West behind the first warm front — the flow shifts Southwest with a brief less wet period through midday Saturday between the warm fronts. Strong West flow aloft develops behind the second warm front early Saturday evening and continues through Saturday night as the surface low moves inland. The strong West flow is what will bring heavy rain to the Cascades. Overall the models have backed off on rainfall amounts in some of the periods this weekend — but not all the models are consistent. Heavy rain will still fall over the mountains with 3-5 inches over the Olympics and 3 to 6+ inches over mainly the Central Cascades in the 36 hours from 4 AM PST Saturday through 4 PM PST Monday — but confidence in the forecast rainfall amounts is lower.

Lastly there could be some locally heavy snow over the Cascades Saturday morning. Cool air from todays upper level trough will drop snow levels down to around 3000 feet early Saturday morning. Snow levels will gradually rise to 5000-7000 feet by late Saturday afternoon as the warm fronts cross the area — but during the transition — up to a foot of snow could fall over Mt rainier and Mt Baker — with possibly up to 6-9 inches around Stevens pass Saturday morning. Of course the same uncertainty regarding rainfall amounts affects snow accumulations.  Winter Weather Advisory may be needed to address the early Saturday snow.

W flow aloft will gradually weaken on Sunday. A convergence zone may develop over Puget Sound on Sunday. Kam


Long-term: — both the GFS and ECMWF have a moderate amplitude upper level ridge forming offshore Monday morning then shifting East over West WA Monday afternoon. Both models also show a surface ridge moving over West WA on Monday — although the GFS is stronger than the ECMWF. This will end the weekends wet windy period. Another weaker warm system moves over the top of the ridge Monday night and Tuesday — bringing a quick end to Mondays brief dry period. The GFS and ECMWF seem to be in fair agreement with the larger pattern past middle next week — but with the models continuing to evolve from run to run — confidence remains low. Bottom line is that active weather will probably be continuing. Kam


Hydrology — though rain will fall at times — flooding is unlikely on any river through Saturday.

Heavy rain could cause flooding by late this weekend. A pair of strong warm fronts will bring heavy rain at times to the coast and Olympics on Saturday. Heavy rain will spread into the Cascades Saturday afternoon but may not be as heavy as initially expected.

Models currently taper rain over the Olympics Saturday night and Sunday — but the stronger West flow aloft Saturday night will bring heavy rain to the Cascades — especially over the Cascades from about King County South. The current rainfall forecast in the 36 hour period from 4 AM PST Saturday through 4 PM PST Monday has 3-5 inches over the Olympics and roughly 3 to 5 inches over the Central Cascades with about 7 inch bullseye over Mt rainier. This is less than the previous forecast. As mentioned in the
Short term: section above — with the evolution of the middle level wind field — confidence in total rain fall amounts is lower.

The snow level will rise to 5000 to 7000 feet by late Saturday afternoon nd night — then lower back to around 5000 feet on Sunday.

These rainfall amounts may be enough to cause flooding. At this time the rivers most likely to flood are the ones flowing off the Cascades of King — Pierce — and Lewis counties — as well as the unusually flood prone Skokomish river in Mason County Any flooding would likely begin as early as Saturday night on the Skokomish river. The Cascade rivers could flood Sunday with crests Sunday night or Monday. Kam



Aviation: — a front over Western WA early this morning was moving inland with rain turning to showers. A stronger frontal system will reach the area later tonight and Saturday. The air will stay moist but there may be some sunbreaks today — then there will be increasing moisture again tonight and Saturday with strong Southwest flow aloft. A deep low pressure area will be in the Northeast Pacific.

SeaTac — rain turning to showers early this morning — some sunbreaks area possible today. Then rain will increase again tonight and Saturday will be wet and blustery.


Marine — a front over Western WA early this morning will move inland. A stronger frontal system will reach the area tonight and Saturday. There are Small Craft Advisories up for most area into this evening — and then overnight tonight and Saturday there will be gales most waters. Coastal seas are expected to peak this evening around 22 ft.


SEW

Watches/Warnings/Advisories

WA —  Winter Weather Advisory through 6 am for the Olympics and Cascades. High wind Watch for the coast and North interior late tonight through Saturday evening. High Surf Advisory for the North and Central Coast from late this morning through early Saturday morning. Coastal Flood Watch for the North and Central Coast from Saturday morning through Saturday afternoon.

Puget Sound — and Northern inland waters Friday night through Saturday.


You can see an illustrated version of this forecast Discussion: at http://www.weather.gov/seattle/gafd/latest_webafd.html



wwus46 Seattle 190456 WSWsew

   URGENT WINTER WEATHER MESSAGE
National Weather Service Seattle WA 856 PM PST Thursday Dec 18 2014



Washington zones: 567>569-191400- /o. Con. Seattle. Ww. Y.0028.000000t0000z 141219t1400z/ Cascades of Whatcom and Skagit counties Cascades of Snohomish and King counties Cascades of Pierce and Lewis counties including the cities of — Marblemount — Concrete — Snoqualmie Pass — Darrington — Index — Randle — Packwood — Ashford — Morton 856 PM PST Thursday Dec 18 2014

 Winter Weather Advisory remains in effect until 6 AM PST Friday —

* some affected locations — mount Baker — Stevens pass — crystal mountain — mount rainier and paradise.

* timing — snow will be heavy at times tonight. Expect snow to taper to snow showers Friday morning.

* accumulations — 6 to 10 inches — heaviest above 5000 feet.

* main impact — travel through the higher passes and on higher mountain highways will require extra caution through Friday morning.

Precautionary/preparedness actions —

Be prepared for slippery roads and limited visibilities. Use caution while driving. For road conditions or travel alerts — call 5-1-1 or 1-800-695-7623 — or visit www. Wsdot. Wa. Gov.




Washington zones: 513-191400- /o. Con. Seattle. Ww. Y.0028.000000t0000z 141219t1400z/ Olympics 856 PM PST Thursday Dec 18 2014

 Winter Weather Advisory remains in effect until 6 AM PST Friday —

* some affected locations — hurricane ridge.

* timing — snow will increase and become heavy at times. Snow will taper to snow showers Friday morning.

* accumulations — 4 to 8 inches — mainly above 5000 feet.

* main impact — hazardous driving conditions.

Precautionary/preparedness actions —

Be prepared for slippery roads and limited visibilities. Use caution while driving. For road conditions or travel alerts — call 5-1-1 or 1-800-695-7623 — or visit www. Wsdot. Wa. Gov.




Recent Winter Weather Statement.

wwus46 Seattle 190456 WSWsew

   URGENT WINTER WEATHER MESSAGE
National Weather Service Seattle WA 856 PM PST Thursday Dec 18 2014



Washington zones: 567>569-191400- /o. Con. Seattle. Ww. Y.0028.000000t0000z 141219t1400z/ Cascades of Whatcom and Skagit counties Cascades of Snohomish and King counties Cascades of Pierce and Lewis counties including the cities of — Marblemount — Concrete — Snoqualmie Pass — Darrington — Index — Randle — Packwood — Ashford — Morton 856 PM PST Thursday Dec 18 2014

 Winter Weather Advisory remains in effect until 6 AM PST Friday —

* some affected locations — mount Baker — Stevens pass — crystal mountain — mount rainier and paradise.

* timing — snow will be heavy at times tonight. Expect snow to taper to snow showers Friday morning.

* accumulations — 6 to 10 inches — heaviest above 5000 feet.

* main impact — travel through the higher passes and on higher mountain highways will require extra caution through Friday morning.

Precautionary/preparedness actions —

Be prepared for slippery roads and limited visibilities. Use caution while driving. For road conditions or travel alerts — call 5-1-1 or 1-800-695-7623 — or visit www. Wsdot. Wa. Gov.




Washington zones: 513-191400- /o. Con. Seattle. Ww. Y.0028.000000t0000z 141219t1400z/ Olympics 856 PM PST Thursday Dec 18 2014

 Winter Weather Advisory remains in effect until 6 AM PST Friday —

* some affected locations — hurricane ridge.

* timing — snow will increase and become heavy at times. Snow will taper to snow showers Friday morning.

* accumulations — 4 to 8 inches — mainly above 5000 feet.

* main impact — hazardous driving conditions.

Precautionary/preparedness actions —

Be prepared for slippery roads and limited visibilities. Use caution while driving. For road conditions or travel alerts — call 5-1-1 or 1-800-695-7623 — or visit www. Wsdot. Wa. Gov.



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
http://lena.jax.org/localinfo/weather/txt/glossary.html
http://www.crh.noaa.gov/dtx/afdterms.php
http://www.theweatherprediction.com/jargon/
METAR interpretation http://weather.cod.edu/notes/metar.html
NOAA's official abbreviations http://weather.cod.edu/notes/abrv.html

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