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Office: MFR

FXUS66 KMFR 132318
AFDMFR

Area Forecast Discussion
National Weather Service Medford OR
318 PM PST Wed Dec 13 2017

.SHORT TERM...Today through Saturday, Dec 16th, 2017...
Little change has occured in the past 24 hours at most locations
today, with a strong inversion remaining across the area. Some mid
elevation ridges on the west side in the 3,000 to 5,000 foot
elevation range have indicated temperatures in the 60 to 70 degree
range, with highs in the 50s all the way up to 8,000 feet on Mount
Shasta and Crater Lake Rim. Meanwhile, cooler conditions remain in
the valleys, with fog and low clouds continuing in and around
Roseburg and Grants Pass.

In the next 24 hours the surface pressure gradient increases from
about 10.5 millibars to 14.5 millibars from Lake County to the
coast. This is likely to result in some increase in east and
southeast wind across the forecast area, mainly at higher
elevations and to the lee of the Cascades and Siskiyous.
Temperatures have raised a little for tonight and, moreso, for
tomorrow due to this increase expected in winds. There is an
outside chance some that some valley locations such as Ashland
could reach 60F Thursday afternoon, though we're going with the
more likely value of 56F.

Model guidance, mainly the GFS, has trended wetter with the cold
front due in on NW flow Friday afternoon and evening.
Precipitation probabilities have been expanded some based on this
recent trend. Snow levels have also been adjusted more toward wet
bulb zero, as models indicate most of the precipitation with this
system being being the cold front. Altogether, amounts are likely
to be light, probably on the order of a tenth of an inch or less
of water. The effect of this system will be to clean out the air
in the valleys and change the temperature regime to more normal,
with temperatures cooler in the mountains than in the valleys.

We expect that the high pressure ridge will rebuild in a weakened
state this weekend, but it's likely to take until Sunday or
Monday for inversions to become established enough that air
stagnation will be a concern again. BTL

.LONG TERM...Sun (12/17) through Wed (12/20)...
It looks like we chose wisely yesterday as the 13/12Z European
Center (EC) model has trended toward the more consistent GFS
solutions in the long range. While there are still significant
differences in the details, the overall pattern evolution looks
similar between the two models through the current forecast
period. Not only that, but both deterministic solutions are
becoming more similar to their respective ensemble solutions,
which is good news.

There is the potential for a significant pattern change late next
week as ensembles show the long wave ridge that has kept us dry
seemingly forever retrogrades offshore and opens the door to more
northerly flow aloft over the PacNW. This pattern would lend itself
to cold air intrusions into the Great Basin which would affect our
east side locations. This pattern would tend to be dry and keep cold
air bottled up east of the Cascades except if and when shortwaves
buckle the flow enough to drive cold air westward and produce any
over water trajectory to moisten things up. Ensembles give us
confidence in the long wave pattern (e.g. cold air on the east
side), but they average out important details like shortwaves (e.g.
cold air moving westward and snow potential). Deterministic models
can often help us with the important details, and that gets better
as we get closer in time.

Bottom line: confidence is growing in a westward shift in the upper
ridge axis and a trend toward colder weather, especially on the east
side toward the latter half of next week. The details on whether any
cold air gets over to the west side or shortwaves that could produce
snow are uncertain, but should get better with time. Prior to that,
weak upper ridging will likely keep our area dry from Sunday through
early Wednesday. Valley inversions are likely to produce stagnant
air in the valleys again during that timeframe. The forecast has
been trended in this direction with the removal of pops, adjustments
to temperatures, and the addition of night time fog. Guidance all
shows a frontal system bringing some chance for precipitation into
the area later Wednesday, and pops have been raised. Although it's
beyond the current forecast period, after Wednesday, the ridge
likely retrogrades bringing colder air to the east side. We'll have
to wait on any more specific details than that. -Wright

&&

.AVIATION...13/18Z TAF CYCLE...Going with a persistence forecast,
current LIFR conditions are expected to persist throughout the TAF
period for portions of the Umpqua Basin, including KRBG, and the
lower portions of the Rogue Valley around Grants Pass. LIFR
conditions at KMFR have improved some and are expected to improve to
VFR within the next few hours.

VFR conditions will prevail elsewhere, including the East Side,
coast, and coastal waters. Similar fog coverage and duration is
expected tonight into Thursday morning with a similar large-scale
pattern persisting. BR-y/SK

&&

.MARINE...Updated 200 AM PST Wednesday 13 Dec 2017...Relatively
light winds for this time of year will persist through the rest of
the week. North winds will persist and will be strongest south of
Cape Blanco, but they are expected to remain below small craft
advisory level. Moderate west swell will build this afternoon
through Thursday and subside Friday, but seas should remain below 10
feet. A weak front will move onshore Friday, followed by offshore
high pressure Saturday. North winds will increase Saturday and could
be strong enough to reach small craft advisory levels in the
southern waters. -Sven/SK

&&

.MFR WATCHES/WARNINGS/ADVISORIES...
OR...Air Stagnation Advisory until 10 AM PST Friday for
     ORZ023-024-026-029>031.

CA...None.

Pacific Coastal Waters...None.

$$

BTL/TRW/MNF



Office: PQR FXUS66 KPQR 132257 AFDPQR Area Forecast Discussion National Weather Service Portland OR 257 PM PST Wed Dec 13 2017 .SYNOPSIS...Little change to the overall pattern through Thursday as a ridge of high pressure remains over the region with low-level offshore flow. The next frontal system moves in off the Pacific late Thursday night or early Friday, bringing a return of onshore flow and chances for lowland rain and mountain snow. This more seasonally typical weather pattern then persists through much of next week. && .SHORT TERM...Tonight through Saturday. Another generally clear day across most of the forecast area, with only some thin high clouds moving over the area as the surface ridge axis reasserts itself over the coastal waters. One notable exception is Hood River and portions of the Hood River Valley where stratus from the Columbia Basin continues persist. These mostly clear skies have resulted in a somewhat warmer afternoon across most of the area - except for Hood River which remains at the freezing mark. Meanwhile, the surface pressure gradient through the gorge continues to slowly increase, as expected - approximately 8.5 mb from The Dalles to Troutdale as of 2 PM. A generally persistence-based forecast should hold through Thursday. Forecast models suggest the gradient could strengthen to around 10 mb by early Thursday morning. While this is not quite as strong as last weekend and early this week, this will still bring a return of windy conditions to these locations. The long-awaited breakdown of the ridge still appears on track for early Friday. Latest model data continues to suggest a slowing of the disturbance as it approaches, so have continued to trend the forecast this direction. As this system is the initial one to move into the longwave ridge, and moisture remains quite modest, will continue to maintain PoPs into the chance category or lower and with relatively little QPF. Offshore flow through the Gorge is likely to continue right up until the time the front moves through Friday morning. As such, the temperatures around Hood River and in the Hood River Valley are likely to remain right around freezing right until the front moves through. Will continue to indicate a slight chance of freezing rain in the Hood River area at onset of precipitation late Friday morning, but confidence is beginning to increase that any period of freezing rain will be very short and with very limited amounts of moisture. It should be noted that if the front does speed up with rain arriving in the pre-dawn time period Friday and before onshore flow breaks the surface inversion, localized patches of freezing rain may be possible to the west (i.e., cooler, more sheltered locations in the northern Willamette Valley north through the Lower Columbia). A few showers may continue through the day Friday behind the front. Shortwave ridging begins to build aloft on Saturday, for an increasingly dry trend across the southern portions of the forecast area. However, maintained some mention of PoPs in far northwestern Oregon and SW Washington as some moisture may attempt to spread over the top of the ridge. Nonetheless, Saturday may end up being a dry day for most. Cullen .LONG TERM...Saturday night through Wednesday. Transient shortwave ridging is quickly replaced by the next disturbance, with precipitation chances back for at least the northern half of the area. However, the models quickly begin to differ in the evolution of these disturbances over the weekend. For the first half of next week, these model differences quickly accumulate to provide for even lower than typical confidence in the details of the pattern that will emerge. Nonetheless, utilizing ensemble data, there is broad agreement in a generally more progressive pattern through long term period (i.e. the first half of next week), and have maintained the forecast that largely follows a climatologically typical pattern. && .AVIATION...Predominantly VFR conditions areawide. High pressure continues to dominate. Stronger offshore flow through the Gorge continues to develops this afternoon through Thu morning, causing an increase in easterly winds, primarily for KPDX and KTTD. KAST might have overnight/ early morning fog, but confidence is low. PDX AND APPROACHES...VFR conditions through 00z Friday. Stronger easterly Gorge winds continue to develop through the night, and into Thursday. /42 && .MARINE...High pressure aloft will bring relatively little change in the weather through the end of the work week. A couple swells moving in from the southwest and west will generally result in seas hovering between 6-9 ft into next week. A relatively weak front will likely bring an increase in southerly winds over the weekend. Expect Small Craft advisory conditions as early as Friday morning, in the northern Oregon, and southern Washington waters. /42 && .PQR WATCHES/WARNINGS/ADVISORIES... OR...Air Stagnation Advisory until 2 PM PST Thursday for Central Willamette Valley-Lower Columbia-South Willamette Valley. WA...Air Stagnation Advisory until 2 PM PST Thursday for Greater Vancouver Area-I-5 Corridor in Cowlitz County. PZ...None. && $$ Interact with us via social media: www.facebook.com/NWSPortland www.twitter.com/NWSPortland This discussion is for Northwest Oregon and Southwest Washington from the Cascade crest to 60 nautical miles offshore. This area is commonly referred to as the CWA, or forecast area.
Office: PDT FXUS66 KPDT 132302 AAA AFDPDT Area Forecast Discussion...Updated National Weather Service Pendleton OR 302 PM PST Wed Dec 13 2017 Updated Aviation Discussion .SHORT TERM...Tonight through Saturday...A ridge of high pressure continues to reside just offshore WA/OR, amplifying slightly this afternoon. Low clouds persist across the lower elevations mainly under 3500 feet, with mostly clear skies above that. The base of the low cloud deck is about 2500 feet above sea level, thus the deck is reaching the ground as freezing fog across portions of north-central OR as far south as RDM, as well as across the southern foothills of the Blue Mountains. With coverage of the thickest fog a bit more limited than the past few days, the freezing fog advisory was allowed to expire at noon today. Think the areal coverage of fog across the aforementioned areas will increase some tonight, but with some uncertainty of dense fog formation, will not reissue the advisory for now. Low temperatures tonight will range from the mid 20s to lower 30s under the inversion, with mainly teems to lower 20s above the inversion. High temperatures Friday will remain in the lower 30s for the lower elevations, with mainly 40s for the higher elevations. Conditions will change little through Thursday Night as the ridge will only slowly weaken with the approach of the next system. Friday/Friday Night...Models are in general agreement with the approach/passage of a trough of low pressure/associated surface cold front. The frontal boundary will move onshore coastal WA/OR early Friday morning, crossing eastern WA/OR Friday afternoon, and exiting to the east Friday Night. The front will not be particularly strong, and given the W/NW trajectory of the system, enough precip shadowing will take place for PoPs to be limited to chance for the Columbia Basin/Blue Mountain foothills, with little/no precip for the Yakima/Kittitas Valleys as well as for most of Central Oregon. Upslope flow across the eastern mountains will yield likely PoPs Friday afternoon/evening. With a relatively weak front, the potential exists for a limited scouring of the low level cold dome. Thus, have trended high temepratures down for the lower elevations, nudging toward the colder ECMWF guidance, which yields mid 30s for the lower elevations. In addition, with the slightly slower movement of the frontal boundary, have delayed the temperature warmup until late afternoon for the lower elevations. This will result in the potential for the initial precip in early/mid afternoon being in the form of freezing rain. With moisture limited, think any ice accumulations will be minor. Snow levels will lower to around 4000 feet Friday afternoon and to around 3000 feet Friday Night, so expect an inch or two of snow across the eastern mountains. The system will exit to the east overnight, with most of the precip ending. Saturday...Ridging will rebuild over the eastern Pacific as the trough exits well to the east. This will result in a chilly NW flow regime across the local area. Enough residual moisture will be present in the upslope flow to wring out a few snow showers across the eastern mountains with snow levels of 1500-2500 feet. Highs Saturday will range from the lower/mid 40s for the lower elevations, to mainly 30s for the mountains. .LONG TERM...Saturday night through Wednesday...A series of weak waves will move through an offshore upper level ridge Saturday through Monday. This will weaken the ridge with time. These waves will keep mostly cloudy skies with rain and snow along the Cascades Saturday night and Sunday. By Sunday night a few showers may develop in the northeast OR mountains. Uncertainty increases for Tuesday and Wednesday. Over the past day GFS runs have varied between deep troughing and flat ridging. The 12Z GFS moves a warm front through WA and OR Tuesday then has a trough/cold front Wednesday. The 12Z ECMWF moves the upper level ridge over eastern WA and OR Tuesday then slowly develops an upper level low offshore with most of the precipitation along and west of the Cascades Wednesday. I kept a slight chance of rain and snow in the mountains Tuesday then increased pops and coverage Tuesday night through Wednesday night as the tendency toward troughing increased in both models. Coonfield && .AVIATION...0Z TAFs...Low level inversions remain across the Interior Northwest this morning. These will persist through the next 24 hours. As such, ceilings at 015-025 will persist through the period at KDLS KPSC and KYKM. IFR conditions at KALW and KPDT will decrease again this evening. VLIFR conditions will persist at KRDM due to weak upslope pressure gradient. KBDN is at the edge of the FG/ST layer. When the weak upslope pressure gradient weakens and the top of the inversion decreases it will push the FG/ST layer back north. LIFR conditions at KBDN may not return tonight. 76 && .PRELIMINARY POINT TEMPS/POPS... PDT 25 29 26 35 / 0 0 0 40 ALW 30 33 30 37 / 0 0 0 50 PSC 31 34 30 36 / 0 0 0 30 YKM 31 32 30 35 / 0 0 0 10 HRI 29 32 29 36 / 0 0 0 30 ELN 29 30 28 36 / 0 0 0 10 RDM 15 31 14 47 / 0 0 0 20 LGD 24 35 27 37 / 0 0 0 40 GCD 27 44 28 44 / 0 0 0 30 DLS 32 35 32 41 / 0 0 0 10 && .PDT WATCHES/WARNINGS/ADVISORIES... OR...Air Stagnation Advisory until 10 AM PST Friday for ORZ041-044- 050-505-507>511. WA...Air Stagnation Advisory until 10 AM PST Friday for WAZ024- 026>029-520-521. && $$ 80/76