"Translated" Seattle area forecast discussion.

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fxus66 Seattle 290427 AFDSEW


Area Forecast Discussion National Weather Service Seattle WA 9:00 PM PDT Thursday May 28 2015


Synopsis: — the Pacific Northwest will remain between a weak upper low to the North over Central British Columbia and an upper ridge to the South. Westerly flow aloft will prevail with generally dry conditions and above normal high temperatures through the weekend. It will be unstable over the mountains with a threat of afternoon and evening thunderstorms Friday. Another upper level low will move into the Pacific Northwest Sunday night and Monday bringing showers and the potential for thunderstorms to much of the forecast area. High pressure will return by middle week.



Short term: — upper level low pressure will shift Southward tonight over Central B. C. With light Westerly flow aloft and stronger low level onshore flow. The air mass remains unstable over the Cascades and Olympics early this evening with the most active area of thunderstorms over the Cascades of Pierce and Lewis counties. While locally moderate to heavy rain is likely occuring with these storms — cells should diminish over the next hour with loss of diurnal heating. The GFS40 forecast sub zero li`s actually encompass areas further Westward Friday afternoon. It could be somewhat active over the Olympics and Cascades — but pushing off to the East early Friday evening as Westerly flow increases. The forecast will be updated to account for mountain thunderstorms.

It was a warm day across parts of interior Western WA with Highs in the low 80s around much of Puget Sound and the Southwest interior. Light Westerly flow kept temperatures cool along the coast — Strait — and around the entrance to the Strait. Onshore gradients are somewhat stronger this evening which should continue into Friday. This should cut down high temperatures a bit across the interior — but less so around the Central Sound Eastward toward the foothills where the warmest Highs near 80 will likely occur. The hrrr shows stratus pushing inland reaching Shelton but not expanding much further inland due in part to increasing cirrus. The last visible picture of the day backs up this idea with stratus not yet pushing through the Chehalis gap.

The ridge builds a bit stronger Northward over the region on Saturday. Onshore flow strengthens and the increased marine influence should knock high temperatures down a few more degrees but still in the 70s over much of the interior. It should be mostly sunny across the interior by afternoon but staying cloudy along the coast.

A transition to unsettled weather will begin on Sunday. An upper low off the Northern CA coast will eject Northeast clipping Western WA. Models differ on timing and details but showers will probably develop across the area during the Sunday through Monday period. The flow aloft will become diffluent ahead of the low on Sunday with the potential for elevated convection above the marine layer over Western WA lowlands. If the upper low comes in negative tilt like the GFS40 shows Sunday night and Monday — dynamic forcing could be sufficient for thunderstorms across Western WA including the lowlands. There is still a lot of uncertainty as to the timing and track of the low so no changes will be Made at this point. Mercer


Long-term: — no changes from previous Discussion: — medium range models continue to highlight building heights over the region on Sunday Sunday ahead of a negative tilted upper level trough approaching from the Southwest. This will likely boost high temperatures Sunday afternoon as well as help pull moisture up from the South and bring an increasing threat of showers late Sunday afternoon and evening over the South — spreading North through the night. The flow pattern is a little more conducive to thunderstorms especially as the upper low will moves inland Monday. Showers and clouds will linger over the area Tuesday and Tuesday night with Highs in the 60s. And Lows in the middle to lower 50s. Weak high pressure aloft will return to the region by midweek bringing drier conditions and a typical pattern of morning clouds and afternoon sunshine.



Aviation: — the air mass is mostly dry and somewhat stable with light Westerly flow aloft. The air mass is more unstable over the Cascades — but early evening thunderstorms will dissipate quickly this evening with loss of daytime heating.

Marine stratus with patchy fog is sitting over the waters of the Pacific. The stratus will push Eastward into the Strait of Juan de Fuca overnight and through the Chehalis Gap to around Shelton and Olympia. The hrrr and GFS show less extensive stratus and fog inland as compared to this morning as cirrus coming into the area from the Southwest inhibits radiational cooling to some degree. Will keep fog and stratus out of the Bellingham — sea — and Seattle/Boeing Field terminals through Thursday morning but continue to show fog in vicinity or tempo stratus at Olympia and pae. Stratus that does occur will be low based and shallow with bases 004-006 msl. Tops will be generally remain below 1000 ft. Visibility restrictions in fog (under 5/8 mile visibility) will occur where the stratus intersects the ground. Stratus will all burn back to the Pacific coastline by 11 AM PDT.

The air mass will be slightly more unstable aloft across the area on Friday than today with lifted indicates -1 to -3 during the late afternoon and evening hours. Moisture and organized lift is lacking — so the threat for thunderstorms will be limited to the Cascade crest area 2 PM PDT 9 PM PDT with only a 10 percent chance that a buildup over the Olympics will give an isolated thunderstorm there. The lowlands are expected to be dry Friday aftn. Albrecht

SeaTac — scattered broken cirrus with North wind 8-10 knots easing to Northeast 5 knots after 12 AM PDT then becoming light and variable in the morning. A Northwest wind to 10 knots will resume after 3 AM PDT. Expect stratus to remain West of the terminal area Friday morning. Albrecht


Marine — high pressure centered offshore with lower pressure inland will give light to moderate onshore flow through the weekend. Small craft advisory conditions can be expected during the late afternoon and overnight hours in the Central and Eastern portions of the Strait of Juan de Fuca.

Areas of fog over the coastal waters will push in through the Strait of Juan de Fuca overnight. Morning fog will be quite patchy in the inland waters during the morning hours on Friday. Albrecht


SEW

Watches/Warnings/Advisories

— WA — none.

Puget Sound Small Craft Advisory Central and Eastern Strait of Juan de Fuca through 6 AM PDT Friday.


Www.weather.gov/Seattle

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
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