"Translated" Seattle area forecast discussion.

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fxus66 Seattle 281723 AFDSEW

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.



— WA — Flood Watches and warnings in effect for portions of Western Washington. High wind Watch western Whatcom — Western Skagit and the San Juan islands tonight and Saturday.

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

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