"Translated" Seattle area forecast discussion.

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fxus66 Seattle 292237 AFDSEW

Area Forecast Discussion National Weather Service Seattle WA 3:35 PM PDT Wednesday Jul 29 2015

Synopsis: — an upper ridge will give dry and hot conditions to the interior of Western Washington through the weekend. An upper trough will bring marine air and cooling to the area starting Sunday night through the middle of next week.

Short term: — the center of a strong and warm upper level ridge over the South Central united states is expanding as it retrogrades toward the four corners region of the desert Southwest through Friday. Flow aloft is light onshore while low level flow is light North to Northwest. A thermal trough at the lower levels is developing from the Olympics Southward through the South interior of Western Washington then into the willamette valley of West Central Oregon.

As the surface pattern develops — 850 millibars temperatures warm to around 21c Thursday through Friday afternoon. With the axis of the thermal trough inland — expect the coast away from the beaches to get warm — but not hot. Areas near the water North of Seattle and in the North interior will be cooled by the 10 to 15 knots North to Northwest breezes keeping Highs in the lower to maybe middle 80s. The Seattle metropolitan area Southward in the interior valleys will get quite hot with widespread Highs in the lower to middle 90s. It will also get hot a few miles East of interstate 5 in the Northern interior from Lynden down to Arlington and Snohomish. Nighttime Lows will fall into the 50s in most rural areas but will stay in the lower 60s in the heat island of the Seattle Tacoma metropolitan areas. It appears that Friday will be the hottest day in the Seattle area with SeaTac airport possibly reaching 94 degrees. Heights and upper level temperatures cool slightly on Saturday with little change in the surface pattern. At this point will go for a 90 degree maximum at SeaTac on Saturday.

Afternoon dewpoint temperatures in the Central Puget Sound air will drop into the upper 40s during the time of maximum heating with the mixing of dry air aloft downward toward the surface with afternoon breezes. So conditions are not expected to get overly oppressive outdoors. But with the hot maximum temperatures and warm overnight Lows — will issue Heat Advisory from midday Thursday until midnight Saturday night for the Seattle — Tacoma — East Puget Sound lowlands — and Southwest interior zones. Albrecht

Long-term: — while the four corners high remains in place through the middle of next week — upper level heights over the Pacific Northwest start a downward trend later Saturday into Sunday for more cooling and possibly a partial inland push of marine air. A full fledged and strong marine push appears to be likely for Sunday night into Tuesday as an upper low moving toward the Alaskan panhandle drops Southeast toward Southern British Columbia and Western Washington midweek. The movement of this upper low appears to be propelled by the development of a weak block along 150W that is being aided by the development of a vertically stacked low near cold bay Alaskan. The 5 AM PDT solutions are in increasingly good agreement and ensemble members are more tightly clustered around the operational GFS solution today. Confidence is still not incredibly high — but the extended forecast will be moved toward this cooler — cloudier — and by Wednesday and Thursday potentially wetter — solution. Albrecht

Climate — on this date back in 2009 the warmest day on record in Seattle occurred with a high of 103 degrees. In addition to being the warmest day on record July 29th is also the driest day on record at Seattle Tacoma airport. Since records started in 1945 there has been a total of just 0.31 inches of rain recorded on this day. Measurable rain has occurred only 4 times ( 0.17 inches 1993, 0.11 inches 2008, 0.02 inches 1955 and 0.01 inches in 1981 ). Felton

Aviation: — an upper ridge will remain over the region. Contd Westerly flow aloft. Low level Northwest flow will become ny late tonight. Visual Flight Rules conditions are expected across much of the area. There will likely be patchy lIFR ceilings/vsbys on the coast after 0600 utc.

SeaTac — Visual Flight Rules. Winds will remain Northerly.

Marine — high pressure offshore with lower pressure East of the Cascades will result in onshore or Northwest flow through Friday. The flow will become briefly Northerly or offshore early Thursday and Friday mornings due to a thermally induced area of low pressure trying to set up along the Washington coast.

Onshore flow will strengthen this weekend — and this will result in the possibility of Gale force winds over parts of the Strait of Juan de Fuca on Sunday. The flow will strengthen further on Monday — leading to an increased threat of Gale force winds over parts of the Strait of Juan de Fuca.

Fire weather:: — a very dry air mass aloft is settling over Western Washington through Friday night. 850-700 millibars layer average relative humidity will be running as low 10 percent between now and then. Mid level haines Index will be solid 5 with values reaching 6 at times over the Central Cascades — indicating a dry and unstable middle level air mass. Will easily reach criteria for the dry and unstable category of Red Flag Warnings through Friday evening. As a result — Red Flag Warning will soon be in effect. It will be targeted to locations above 2000 feet elevation — which will stick up above the inhibiting effect of low level subsidence inversions. With such a dry air mass — leftover fuel moisture from recent rains will quickly become a memory. Over the mountain zones — this kind of air mass supports active fire behavior and rapid growth on existing fires. Haner



— WA — Heat Advisory midday Thursday through Saturday evening for the Seattle area — the Tacoma area — the East Puget Sound lowlands — and the Southwest interior.

Red Flag Warning in effect through Friday evening for a dry and unstable air mass in
Fire weather:: zones 652 — 658 — 659 and 661.

Puget Sound — small craft remains in advisory until 11 PM PDT tonight for the coastal waters from James Island to cape shoalwater out to 10 nm.


An illustrated version of this forecast Discussion: can be seen 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