Marine Weather and Tide Forecast for Grants Pass, OR

FXUS66 KMFR 281128 AFDMFR

Area Forecast Discussion National Weather Service Medford OR 427 AM PDT Mon Apr 28 2025

SHORT TERM
High pressure will build into the area today with dry weather. Onshore flow and a marine push into the Umpqua Valley is bringing cloudy skies to much of Coos, Douglas and northern Curry counties this morning, but no rain. The depth of the moist layer extends up to around 3500 feet, and this is well-shown by the latest nighttime microphysics RGB satellite imagery, which shows this stratus banking up against the Rogue-Umpqua Divide.
South of there, clouds are patchy at best. There is a chance some of the stratus "spills over" into the Rogue Basin (banking up against the Siskiyous) toward sunrise. Even if it does, these clouds will be short-lived, break up into stratocumulus later this morning and then scatter out this afternoon yielding sunny skies for most. It'll be milder than yesterday with MaxTs in the upper 60s and low 70s for most west side valleys (75-80F in the lower Klamath Valley from Happy Camp to Somes Bar) and in the 60s over the East Side. Overall, fairly typical afternoon N-NW breezes can be expected area wide that'll last through early evening.

A weak trough will move southeastward from the NE Pacific tonight and swing through the area on Tuesday. This will bring some clouds and slight chance to chance PoPs (~15-50%) for showers from the Coast to the Cascades and over the East Side (best chance is in the mountains). If it rains at all, amounts will be light, generally only a hundredth or two. This system will exit to the south and east of the area Tuesday evening with high pressure re- establishing Wednesday into Thursday. Expect cool (west side) to chilly (east side) mornings both days, but with nice, warm afternoons. Peak of the warmth is expected on Thu with highs in the valleys west of the Cascades in the 80-85F range (upper 80s lower Klamath). East Side locations will have highs largely in the mid and upper 70s.

Next trough is still slated to arrive Friday into next weekend.
Details to follow in long term section below. -Spilde

LONG TERM
On Friday, a deeper negatively tilted trough is anticipated to develop with the trough axis extending down through the PacNW from British Columbia. The base of the trough and vorticity maximum will pass over the forecast area. This coupled with with ample CAPE (upwards of 600 J/kg) will likely lead to showers and isolated thunderstorms. However, it should be noted that instability has decreased in areal coverage, with instability now being confined to areas along and near the Cascade, and areas to the east of the Cascades (including northern California. Bulk shear values (also decreased) around 30-40 knots would allow for updrafts to maintain, so we could have a couple strong storms; however, its too early to say with much confidence the coverage of these thunderstorms given fluctuations in upper level dynamics. DCAPE values remain on the lower end, and this is evident with forecast soundings indicating moist low levels. In fact, extended range forecast soundings show a "tropical" like soundings with a very moist column. Will continue to monitor this because Friday could be impactful from a rain and lightning point of view, but small hail cannot be ruled out either at this point.
PWATs are approaching an inch in some areas, so we will need to also keep an eye on heavy rainfall potential. Early indication suggest the system is progressive, and the mean flow could result in fast moving cells which would limit rainfall amounts over one area. We will continue to monitor Friday for impacts as this remains the day with the highest potential.

-Guerrero
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