Area Forecast Discussion
Issued by NWS Pendleton, OR
Issued by NWS Pendleton, OR
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052 FXUS66 KPDT 161622 AFDPDT Area Forecast Discussion National Weather Service Pendleton OR 922 AM PDT Tue Jul 16 2024 .MORNING UPDATE...No real update to the forecast. However, a Red Flag Warning has been issued for the southern Blues and Strawberry Mountains for dry and unstable conditions. With the fire already on the landscape, these conditions will cause the fire and any new fires to exhibit extreme fire behavior. Bennese/90 && .PREV DISCUSSION... /issued 444 AM PDT Tue Jul 16 2024/ SHORT TERM...Today through Thursday night...A compact vorticity maximum that is currently approaching northwest CA and southwest OR will be the main focus for weather this afternoon through Wednesday as it tracks north-northeastward across western Oregon this evening and overnight before ultimately exiting the region via northeast Washington on Wednesday night. Confidence is very high (>80% chance) that some high-based convection will develop this afternoon across central Oregon as a result of synoptic forcing and moisture advection associated with the incoming shortwave coupled with modest (<500 J/kg) MUCAPE advertised by the 00Z HREF. A deep, well-mixed boundary layer will increase the likelihood of dry lightning and gusty outflow winds. This evening, as the wave tracks northeastward, mid- to upper-level forcing coupled with increasing PWATs should facilitate at least isolated to scattered high-based showers (>50% confidence), though CAMs are still exhibiting some run-to-run and model-to-model differences regarding how robust the convection will be. Current thinking is lightning will be limited overnight, though any that occurs will likely not (>90% chance) be accompanied by a wetting rain. Instability ramps up along the east slopes of the Washington Cascades Wednesday morning and afternoon, and forecast soundings show more robust MUCAPE of 500-1000 J/kg, locally higher. Confidence in isolated to scattered thunderstorms is highest there. What remains in question is how much rain will accompany convection. Forecast PWATs are certainly high enough to support a wetting rain with some cells. Further east across the Blue Mountains of northeast Oregon and far southeast Washington, uncertainty in convective potential is high due to a wide spread in the forecast surface moisture field. HREF members suggest dew points ranging from roughly 35 to 55 degrees. Should the higher moisture materialize, convection would initiate along the Blues Wednesday afternoon, subsequently diminishing Wednesday evening. Have opted to nudge PoPs over the NBM`s values, but have not included a mention of showers or thunderstorms due to low confidence. Continued heat has prompted additional heat highlights across many of our low-elevation zones today through Wednesday or Thursday. Cloud cover Wednesday night should limit radiational cooling such that overnight lows remain warm enough to push HeatRisk values to Major across the majority of the Yakima and Kittitas valleys. In response, have opted to upgrade the existing Heat Advisories into an Excessive Heat Warning, valid noon today through 10PM Thursday. Elsewhere, have included the John Day Basin in a Heat Advisory from noon today until 10PM Wednesday. Thursday, locally breezy westerly winds will develop through the Cascade gaps and into the Columbia Basin as another shortwave trough approaches the PacNW. Confidence is currently low (<50% chance) in any wind or fire weather highlights on Thursday. Plunkett/86 LONG TERM...Friday through Tuesday...The primary sensible weather concerns revolve around heat concerns this weekend and elevated fire weather conditions. The latter for: a) dry and windy conditions Monday across Cascade gaps/east slopes, the eastern Gorge spilling into the Lower Basin, and Kittitas Valley and b) very low isolated thunderstorm potential mainly late Sunday and Monday, albeit with appreciable spread in timing/details. An amplified large scale pattern is forecast early Friday that will be characterized by a passing embedded mid-level shortwave trough lifting northeastward across the PacNW with a longwave ridge anchored across the Desert Southwest extending into Nunavut and Northwest Territories. Ensemble guidance is in fairly good agreement with the synoptic evolution through Sunday. Of which, the strong amplified mid-level ridge will evolve westward to be move over NV extending north into the Northwest Territories by Saturday and Sunday with an upper trough/closed low upstream in the eastern North Pacific. Impressive ECMWF mean 500 mb heights forecast with mean heights forecast outside model climo across Canada with interior PacNW heights exceeding the 90th percentile. After a brief decline in highs on Friday owing to a weak shortwave trough and its associated front that hangs in the western NW, oppressive heat is anticipated to build back across the interior NW this weekend. Increased confidence in building heat buttressed by increasing 850 mb temperatures, ample insolation, dry air mass, and deep mixing. ECMWF mean 850 mb temps forecast to exceed the 97th percentile Saturday and 99th percentile Sunday. Of which, warmest 850 mb temperature anomalies are centered over eastern WA, northeast OR, and northern ID extending into Canada. Confidence in heat further supported by Extreme Forecast Index values of 0.6-0.9 indicative of a large number of raw ensemble members forecasting warmer than average highs with many extreme forecasts in excess of 110 F in the lower elevations. Latest NBM guidance show odds for 100 F or more in excess of 75% across large parts of south central WA into the lower Basin, foothills of the Blues, and John Day. Odds for 110 F are limited Saturday with the greatest seen Sunday between 15-25% across parts of the lower elevations. NWS HeatRisk is showing widespread Major (category 3) Saturday and Sunday, the latter day appearing the most impactful driven by mild, warm lows with upper 60s forecast. Of which, if underdone, lower 70s would increase the risk to sensitive groups and at-risk populations. Start to see increasing differences Monday onward with clustering scenarios showing the main source of spread/uncertainty revolving around the upstream trough, its eastward progression/shunting of the upper ridge, and possible embedded open waves. An initial shortwave trough embedded in the flow is possible to rotate late Saturday or Sunday, however, significant differences exist with gudience in terms of timing. Confidence in this open wave disturbance is not well supported (~15% confidence) Saturday with a little more support in its occurrence Sunday (~25% confidence). That said, moisture will be a limiting factor but the combination of dry fuels/antecedent conditions with isolated thunder risk bears mentioning. The greatest risk area is across the upper slopes of the WA Cascades and central OR Sunday. The low chances precludes a mention in the forecast at this time. Thereafter there is greater potential for a disturbance affecting the area on Monday (~40% confidence) based on ensemble clustering with greater support amongst ensemble members. This will promote very low potential for thunderstorms as well, though with the threat area spreading eastward across the eastern mountains as well. Furthermore, increased potential for dry and windy conditions are anticipated across the eastern Cascade gaps, eastern Gorge spilling into the Lower Basin, and Kittitas Valley. 24-hr chances for peak daily gusts greater than 39 mph exceeds 50% across the aforementioned areas, except near 80% across Kittitas valley. While there is growing confidence in a reprieve in temps from this weekend by Tuesday (60-70%), there is still potential that the upper troughs progress will be slow that would help the heat linger into early next week. AVIATION...12Z TAFs...VFR conditions area-wide presently with some passing mid-clouds across northeast OR and south central WA. Smoke has been noted at KJSY and KALW overnight. Outside haze, mostly clear skies across the area until this afternoon as a disturbance promotes increasing high-end VFR, mid- and high-level clouds across central OR spreading across the remainder of the area overnight. There is low potential for isolated thunderstorms this afternoon and evening at central OR terminals. This threat then spreads to KDLS and KYKM during the small hours-tomorrow, and more so the KPDT and KALW tomorrow afternoon/evening. However, low confidence precludes a mention in the TAFs at this time. Otherwise, winds will be light (less than 12 kts) with a weak surface pressure differences in place. Otherwise, surface smoke may be seen across northeast OR but confidence in sub-VFR visibility is low (less than 50%) based on latest HRRR forecasts. FIRE WEATHER...Fire weather watches remain in effect for much of central and north-central Oregon as well as south-central Washington due to the potential for dry thunderstorms late this afternoon, overnight, and Wednesday. While locally breezy conditions are forecast, especially Wednesday and Thursday through the Cascade gaps, we are not currently forecasting overlap of critical wind and RH. && .PRELIMINARY POINT TEMPS/POPS... PDT 99 65 100 65 / 0 0 20 0 ALW 102 67 103 68 / 0 0 20 0 PSC 102 67 102 68 / 0 0 20 0 YKM 101 68 100 65 / 0 10 20 0 HRI 103 67 104 68 / 0 10 20 0 ELN 101 66 100 66 / 0 0 20 20 RDM 98 60 96 55 / 10 20 0 0 LGD 98 61 97 60 / 0 0 20 10 GCD 98 64 99 58 / 0 20 20 0 DLS 105 70 99 67 / 0 20 20 0 && .PDT WATCHES/WARNINGS/ADVISORIES... OR...Heat Advisory until 10 PM PDT Wednesday for ORZ041-505-507-508- 510. Fire Weather Watch from 5 PM PDT this afternoon through Wednesday afternoon for ORZ610-611-639-640. Heat Advisory until 10 PM PDT Thursday for ORZ044. Red Flag Warning from 2 PM this afternoon to 11 PM PDT this evening for ORZ642. WA...Heat Advisory until 10 PM PDT Wednesday for WAZ024-029-521. Heat Advisory until 10 PM PDT Thursday for WAZ028. Fire Weather Watch from this evening through Wednesday evening for WAZ690-694-695. Excessive Heat Warning until 10 PM PDT Thursday for WAZ026-027. && $$ SHORT TERM...86 LONG TERM....80 AVIATION...80