Subj : HVYRAIN: Excessive Rainfa To : All From : Mike Powell Date : Tue Dec 12 2023 08:40:00 FOUS30 KWBC 120815 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 315 AM EST Tue Dec 12 2023 Day 1 Valid 12Z Tue Dec 12 2023 - 12Z Wed Dec 13 2023 The probability of rainfall exceeding flash flood guidance is less than 5 percent. Kleebauer Day 2 Valid 12Z Wed Dec 13 2023 - 12Z Thu Dec 14 2023 ...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF SOUTHEAST FLORIDA... Models continue to indicate a period of convective development across the southeastern tip of the FL Peninsula in conjunction with a weak mid-level impulse moving up from the Western Caribbean in tandem with a stationary boundary over the FL Straits moving back north in time for Wednesday afternoon/evening. There is some suggestion of a little less vigor in the convective potential just due to the changes within the mid-level energy advection and the northern extent of greater theta-E's in association with the increased low-level moisture flux along our frontal boundary. The latest HREF isn't super enthused with the potential, but there's still some hints of isolated heavy thunderstorms possible within the frictional convergence field established near the SE FL coast as energy moves overhead with peak diurnal destabilization. The most aggressive models are actually showing localized 3-5" maximums within a short window late-Wednesday afternoon into the early evening before giving way to some passing showers and isolated thunder for the remainder of the period. Considering the environmental parameters shifting towards a favorable convective pattern within a significant moisture flux, opted to maintain the previous MRGL, but did tighten the bounds on the northern end as the best potential is focusing between Fort Lauderdale down to Miami/Homestead. Kleebauer Day 3 Valid 12Z Thu Dec 14 2023 - 12Z Fri Dec 15 2023 ...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS SOUTHEAST FLORIDA AND WEST TEXAS... ....Southeast Florida... Convergence pattern focused along a stalled frontal boundary extending through the southern FL Peninsula will lead to a continued convective pattern within the confines of the same area that will be impacted on D2. A bit more vigor anticipated within the time frame as the pool of low-level instability from the nearby Straits and northern Caribbean will be firmly planted across the southern tier of FL. With strong surface ridging to the north providing an incessant easterly fetch and frictional convergence regime, localized heavy rainfall will be possible from any thunderstorm development within the coastal plain. Instability wise is a bit better compared to the previous period, so this aligns well with the opportunity for waves of convection to impact the urban setting between West Palm down to Miami. Deterministic output has some fairly decent "bullseye" markers in the QPF field with 24 hr precip totals on even the NBM flirting with approximately 2" in the time frame. That's pretty decent considering the lack of hi-res deterministic model input within that time scale of the NBM. The setup is still one for monitoring on upgrades (SLGT risk) in future updates pending the hi-res signatures in later model runs. ....West Texas... A deep, closed upper-low centered over the Southwestern U.S will slowly push eastward Thursday into Friday with a strong difluent signature over NM into the western half of TX. Deep ascent and backed flow at the surface will allow for a migration of elevated moisture to come surging north in time to interact with an intensifying QG convergence pattern centered over the southern high plains. Precip will blossom ahead of the mean trough with mid-level vorticity advection proposing an extended period of elevated convective concerns east of the Caprock, mainly east and south of Amarillo and Lubbock. PWAT anomalies running 2-3 standard deviations above normal within the zone of strong ascent will capitalize on the pattern to produce locally heavy rainfall in the aforementioned areas despite some fairly cool/cold temperatures at the surface. The key element with the setup will be the training factors of the precip development across the high plains with an alignment of south to north QPF forecasts across all major deterministic and ensemble averages. This isn't a classic flash flooding threat due to the limited surface instability elements, but the upper-level pattern is one that would yield a more significant precip threat, so have maintained the MRGL risk from previous forecast(s) and will assess exact placement of where the elevated convective threat and training pattern could occur as QPF forecasts are already exceeding 2" with increasing probabilities of at least 2" within the ensemble guidance (ECENS, GEFS, and NBM). Kleebauer $$ --- SBBSecho 3.14-Linux * Origin: Ilink: CCO - capitolcityonline.net (454:1/105) .