Area Forecast Discussion
Issued by NWS Denver/Boulder, CO
Issued by NWS Denver/Boulder, CO
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059 FXUS65 KBOU 201757 AFDBOU Area Forecast Discussion National Weather Service Denver/Boulder CO 1157 AM MDT Fri Sep 20 2024 .KEY MESSAGES... - Dry today with above normal temperatures. Breezy and areas of elevated fire weather conditions - mainly over the Palmer Divide to Park County. - Storm system will impact the area this weekend. 80-90% chance of precipitation, with rainfall amounts the last remaining uncertainty. The first real mountain snow is likely, with >70% probabilities of accumulating snow above 9,000ft. - Turning much cooler by late Saturday through Sunday. Below normal temperatures early next week, then likely (60% chance) back to drier and warmer weather toward the end of next week. && .UPDATE... Issued at 1037 AM MDT Fri Sep 20 2024 Still pretty quiet in the short term. There is an enhanced band of upper level cloudiness along the southern fringe of the upper level jet, and this feature moves very little today so expect a continued but fairly narrow stream of high clouds. This could keep temperatures down a couple degrees along/north of a Denver to Sterling line. Will make a little adjustment in the sky and forecast max temperatures today to account for that. Forecast is on track with breezes, warmth, and dryness for elevated fire weather conditions over the Palmer Divide to Park County. && .SHORT TERM /Through Tonight/... Issued at 341 AM MDT Fri Sep 20 2024 Southwest flow aloft continues over northeastern Colorado as our upper level ridge pattern ends today. Cross sections indicate weak mid-level moisture resulting in a few clouds for the eastern plains. With limited moisture and a lack of instability, precipitation is not expected today. 700mb temperatures between 12-14C should lead to above normal temperatures this afternoon. Afternoon highs range between low to upper 80s for the plains and low 60s to low 70s for the mountains and valleys. Near critical fire weather conditions are possible mainly along the Palmer Divide and adjacent plains due to low relative humidities in the teens and breezy wind gusts up to 20 mph. Tonight, near normal lows are expected and surface winds increase as the next system approaches. && .LONG TERM /Saturday through Thursday/... Issued at 341 AM MDT Fri Sep 20 2024 The main focus of this period continues to be this weekend`s rain and snow event. As we have messaged in the past few AFDs, there remains considerable uncertainty in the forecast (mainly QPF), despite growing agreement in the upper level features of this system. In general, there has been a slightly southward/drier shift across the 00z model suite, though there are still quite a few very wet solutions offered by tonight`s guidance. The synoptic pattern at this point is pretty well established. An upper level low - currently centered near the southern California coastline - is expected to drift into the desert southwest, then lift northeastward from the Four Corners region into eastern Colorado from Saturday into Sunday. Models are in good agreement that this upper low will gradually weaken and transition to an open wave as it begins to be picked up by the northern stream flow. The mid-level low should also take a similar track, though it`s also weak and as a result the upslope flow isn`t particularly strong or long-lasting across our CWA. At the surface, a strong cold front should be moving across the forecast area early Saturday morning, with temperatures on Saturday significantly cooler than what we`ve seen in the last week or two. Forecast high temperatures will likely peak in the upper 60s to 70s across the plains, which would be the coolest temperatures so far this month. As a surface high strengthens across the northern plains, it should promote continued cold air advection/shallow upslope flow through much of the day. Increasing synoptic scale lift will eventually lead to widespread precipitation developing Saturday afternoon or evening, with the bulk of precipitation expected to fall Saturday night into the first half of Sunday. Sunday will be the coolest day in quite some time, with highs likely struggling to get out of the 50s across the plains. Showers should come to an end as the storm system shifts off to our east and our source of lift diminishes. Like the day shift, I am similarly perplexed by the lack of consistency in the QPF forecast. There are no clear trends to work with, and each of the main ensemble systems still have quite a bit of spread. The most recent ECM ensemble for Boulder still has a mean of just over 3/4" of liquid, though there is one member with a trace, and a handful of members with >2" of rain over a 24 hour period. Our forecast lies a little on the wetter side of guidance but is overall a touch drier than the previous forecast. In terms of snowfall, the forecast hasn`t changed significantly. We still expect several inches of snow above 10kft with snow levels dropping to as low as 8500-9000ft. Again, we did consider a Winter Weather Advisory for a couple mountain zones but held off due to the unclear QPF forecast. We`ll punt that decision back to the day shift and hope today`s guidance offers a little more clarity. We should see temperatures near normal (or perhaps a little cooler) through the first half of next week, with northwesterly flow aloft persisting. Models generally have one or two shortwaves dropping down into the region on Monday/Tuesday, with hints at another strong cold front for Tuesday. There would be a chance for additional rain showers and mountain snow - albeit a lower chance than this weekend - but at the very least a return of consistent 80/90 degree days looks unlikely. Model solutions diverge as we get later into next week, but the general pattern would favor a return to warmer and drier conditions. Both the GFS/GEM build up a strong ridge by the latter half of the week, while the ECM cuts off one of the previous shortwaves, which then aimlessly drifts across the mountain west. The broader ensemble still favors a return to above normal temperatures and the NBM provides a reasonable starting point for this period. && .AVIATION /18Z TAFS through 18Z Saturday/... Issued at 1157 AM MDT Fri Sep 20 2024 VFR conditions continue through the TAF period. Winds are expected to hold from the ESE through the afternoon at KDEN and KBJC, with a shear zone/cylone development this afternoon to the south. There is relatively high confidence this shear zone stays south (70-80%) through the afternoon. However, KAPA could be just far enough south for gusty south/southwest winds there (50-60% confidence) toward peak heating/mixing around 21Z. Winds get a little more tricky through this evening, with the shear zone/cyclone potentially lending toward more north/northwest flow 01Z-05Z, and then disrupted or lighter than normal drainage winds overnight. We`ve opted for a middle of the road approach with more westerly component overnight. A cold front arrives 12Z-14Z Saturday with switch to northerly winds. Low level moisture is rather scant behind the front, so the threat of any stratus or lower ceilings is still small (<20%). Showers will increase after 21Z Saturday with gradually lowering cloud decks then. && .FIRE WEATHER... Issued at 341 AM MDT Fri Sep 20 2024 With relative humidities dropping to 11-15 percent this afternoon, near critical fire weather conditions are possible mainly for Lincoln county and the Palmer Divide. Southwest winds remain below critical thresholds producing wind gusts up to 20 mph through early evening. && .BOU WATCHES/WARNINGS/ADVISORIES... None. && $$ UPDATE...Barjenbruch SHORT TERM...AD LONG TERM...Hiris AVIATION...Barjenbruch FIRE WEATHER...AD