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Area Forecast Discussion
Issued by NWS Medford, OR
Issued by NWS Medford, OR
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775 FXUS66 KMFR 151743 AAA AFDMFR Area Forecast Discussion National Weather Service Medford OR 1043 AM PDT Thu Aug 15 2024 .DISCUSSION...The upper low that brought abundant thunderstorms to mainly the east side continued to exit northeast out of the area overnight and early this morning. A few rain showers remain over northern Lake and Klamath counties this morning...but will move out within the next hour. Otherwise...the focus of the forecast today will be thunderstorm potential on the east side. The short term models run after run continue to shift the initiation east...with confidence diminishing as far as the scattered nature of thunderstorms. We will be evaluating and make any necessary edits within the next few hours. Once we get through today...we will be taking a much closer look at the low forecast to impact the region over the weekend. Confidence continues to increase that areas along and west of the Cascades will see wetting rain. More to come...stay tuned! -Riley .AVIATION...15/18Z TAFS...Ceilings at the coast and across coos and coastal Douglas Counties will be VFR for much of the day before returning to to MVFR late evening and overnight. Guidance is suggesting around a 30 percent chance for IFR stratus, but left it out of the TAF due to the unsettled nature of the pattern. Thunderstorm chances this afternoon and evening are generally limited to Lake and Klamath counties, with 20% chances at the southern end and 40% chances along the northern edge of these counties. Extra caution is advised around any storms that do develop, as thunderstorms can generate lightning, gusty erratic winds, and locally lower ceilings and visibilities. Chances for activity drop to single digits late tonight. Wildfire smoke also continues to complicate conditions. Winds will generally come from the west and brief periodic moderate to heavy smoke over western Siskiyou, northern Lake, northern Klamath, and eastern Douglas counties with MVFR conditions in their vicinity and downwind. Light to moderate smoke is possible over most inland counties through the TAF period. -Schaaf/TAD && .PREV DISCUSSION... /Issued 459 AM PDT Thu Aug 15 2024/ DISCUSSION...It was an active evening on radar with isolated to scattered thunderstorms that continued until around midnight. During this time, around 1200 lightning strikes have been recorded...most numerous across Klamath and Lake Counties, but also in portions of central and eastern Siskiyou County. Rainfall amounts with these recent storms have been rather impressive. 0.25" is a common report at observation sites across these areas, with even reports of 0.50" occurring at several locations. Thunderstorm activity has significantly dwindled as of this writing (~430am), but there are some lingering showers across far northern Klamath County in the Diamond Lake/Chemult area and northward. Dry conditions persist for the remainder of the area with satellite imagery showing some mid-level cloud cover over much of the West Side. This shower/thunderstorm activity was brought to you by a shortwave trough passing through the region yesterday evening, which was the trigger needed to tap into some elevated instability and moisture. This trough will linger over Oregon today and, after a break in activity through this morning, it will reinvigorate shower/thunderstorm activity this afternoon and evening. The focus of storms today will be shifted east and northward, with the best chances (25-40%) expected over Klamath and Lake Counties north of Highway 140 where high resolution guidance continues show another round of isolated to scatter thunderstorm activity. The one "fly in the ointment" leading to some uncertainty in the extent of thunderstorm development will be the amount of smoke expected east of the Cascades today. Given that thunderstorm development today will largely be surface based, smoke could be a limiting factor as it can act as a cap on surface based instability. In the event that surface heating is able to overcome the limiting effects of smoke, have continued the Red Flag Warning for portions of Fire Weather Zones (FWZs) 624 and 625 for abundant lightning on dry fuels for today, and details can be found at RFWMFR. The atmosphere is more stable on Friday. Lingering moisture and weak instability will be present, but there isn`t a defined trigger coming through the region, so thunderstorm chances largely drop out of the forecast for Friday. It`s likely there will be some cumulus buildups east of the Cascades and over the higher terrain and there`s a small chance (5- 10%) a few of those buildups could result in a brief shower east of the Cascades. For the remainder of the area, dry conditions will continue along with below normal temperatures (afternoon highs in the mid to upper 80s for West Side Valleys and upper 70s to low 80s for areas east of the Cascades). Despite the reprieve from the summer heat, smoke will be a continued nuisance through the forecast period, and probably longer. An Air Quality Alert (issued by partnering agencies) is in effect for Klamath and Lake counties, and we have relayed those via AQAMFR. The cooler than normal pattern will continue as a deep, large scale trough develops off the PacNW Coast late Friday into Saturday and lingers there well into next week. The center of the low will be at it`s southern most location on Saturday before it drifts northward Sunday into early next week. As such, Saturday will be the day with the highest precipitation potential through at least mid-week. Confidence is increasing on the potential for measurable precipitation (even possibly wetting rains) for much of the area along and west of the Cascades as ensembles are in remarkable agreement. When looking at the chance of total precipitation amounts ending at 5 pm Saturday (for areas along and west of the Cascades), there is a 80-90% chance of at least 0.01" (measurable precipitation), and about a 40-60% chance of at least 0.10". For wetting rains along and west of the Cascades, there is about a 25- 30% chance of 0.25". Even locations in northern California from the Shasta Valley westward have similar chances, albeit lower chances for 0.10" (30%) and 0.25" (10%). Models indicate ample instability to go with this moisture so there is moderate confidence in thunderstorms occurring west of the Cascades on Saturday. The uncertainty lies in how much lightning there will be...will it be a convective event with abundant lightning or will it be a broad area of showers with embedded thunderstorms. Once this timeframe is resolved by high resolution models, we`ll evaluate the need for any fire weather products. Uncertainty increases for areas east of the Cascades regarding precipitation chances. There will likely be a sharp gradient between where precipitation chances drop off and the concern for strong winds combined with low humidities results in critical fire weather conditions. Current guidance suggests this gradient will be along and near the Warners, extending northward to Winter Rim/Summer Lake area. West of that area, the chance for 0.01" of precipitation is about 50%, while the chance for 0.10" drops to 15% and only a 10% chance for 0.25". Farther east towards Lakeview and Alturas, the chance for 0.01" of precipitation is only 10% with no solutions indicating a chance for higher amounts. The bigger concern east of the Cascades (and where the higher confidence lies) will be strong south to southwest winds. Guidance is currently showing the potential for gusts of 30-40 mph east of the Cascades and in the Shasta Valley. There will likely be areas where these strong winds combine with low humidities and result in critical fire weather conditions. Exactly where and how low humidities fall is where the uncertainty lies at this point in time, and will ultimately depend on where precipitation chances occur. Right now, it looks like the best chances for critical fire weather conditions exist over much of FWZ 625 and portions of the Modoc in FWZ 285. Stay tuned as we refine the details over the coming shifts. From Sunday onward, low pressure will drift northward and precipitation chances will come to an end. We`ll remain under a cooler than normal pattern with gusty afternoon breezes. Shortwaves will rotate around this low pressure and into the region through the forecast period, though the timing of these are difficult to pinpoint at this time range. Stay tuned and monitor for updates as these shortwaves could result in either enhanced afternoon winds (possible fire weather concerns) or thunderstorm chances at some point next week. /BR-y && .MFR WATCHES/WARNINGS/ADVISORIES... OR...Red Flag Warning until 11 PM PDT this evening for ORZ624-625. CA...None. PACIFIC COASTAL WATERS...None. && $$