Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Tue Feb 27 2024 08:03:40 FOUS30 KWBC 270803 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 302 AM EST Tue Feb 27 2024 Day 1 Valid 12Z Tue Feb 27 2024 - 12Z Wed Feb 28 2024 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE OHIO AND MID-MISSISSIPPI VALLEYS AND THE CENTRAL APPALACHIANS... Two main features to note via WV satellite; digging shortwave trough over the northern Rockies will continue to propagate to the southeast, eventually shifting east, then northeast by the end of the forecast cycle. Out ahead of the mean trough out west, a fast moving vort max will motion out of the mid-Mississippi Valley overnight and push through the Mid-Atlantic and Northeast U.S by morning, exiting out of the picture by the afternoon hours. These two disturbances will be the primary rain makers for the eastern half of the CONUS Tuesday into Wednesday morning. The bigger impact will be from the former of the two disturbances as the shortwave progression out of the Rockies into the plains will begin to ramp up the convective pattern east of the Mississippi. Broad difluent flow ahead of the shortwave trough will allow for greater large scale forcing to take shape within a priming environment thanks to boundary layer flow out of the south ushering warm, moist air poleward from the Gulf. Increasing instability axis in-of the Tennessee and Ohio Valleys will be setup for the incoming shortwave trough as theta-E fields signal a strong meridional push of low-level moisture with sights on areas as far north as the Great Lakes by later this afternoon. A surface low will spawn within the central Midwest with a triple-point depiction fairly consistent on deterministic guidance, especially as it matures heading into northern IL. By late afternoon, scattered showers and storms will fire within the warm sector and quickly move to the northeast as the steering flow is fairly progressive through the period. Locally heavy rain can be found within any of these convective cells, but the forward motion of the showers/storms will likely mitigate the flash flood threat to a low-end probability when coupled with the expected rates (0.5-1"/hr based off latest HREF neighborhood probability fields). As we step forward into Tuesday evening through the overnight time frame, more showers and storms will organize into a tight linear segment just out ahead of a swift-moving cold front plowing eastward out of the Mississippi Valley with greater coverage downstream over the central Appalachians up through western PA thanks to increasing difluent pattern pivoting eastward with the progression of the mean trough. QPF forecast shows local maximums within the confines of central and southern OH into the previously stated zones, mainly due to greater convective clustering as cells merge over time after their initiation over places further west. Some hi-res deterministic are being fairly liberal with their QPF footprint indicating some local 1.5-2" totals within the confines of the Alleghanies in western PA, back into southern OH thanks to the organized convective pattern. Elevated PWAT indices approaching 2-3 deviations above normal would constitute an environment favorable for locally heavy rain, and with the topographic elements coming into play within those areas, that is where the best opportunity for localized flooding concerns is anticipated. The saving grace is the progressive propagation of the system as a whole which will limit a larger scale event and keep this closer to a Marginal Risk which has been the forecast for the past several cycles, and will be maintained in this forecast package. The main adjustments were related to the latest QPF footprint of the CAMs and alignment with the favorable instability fields which will drive the rates and convective regime necessary for flooding to occur. Kleebauer=20 Day 2 Valid 12Z Wed Feb 28 2024 - 12Z Thu Feb 29 2024 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE UPPER OHIO VALLEY THROUGH NEW ENGLAND, AS WELL AS THE COASTAL PACIFIC NORTHWEST... ....Mid-Atlantic and Northeast... Progressive, but potent shortwave trough will eject east-northeast while evolving from a neutral to negative tilt as it crosses east of the Mississippi. A strong cold front will press east through the Ohio Valley into the Mid-Atlantic and Northeast by the second half of the forecast period. A line of organized showers and embedded thunderstorms will rumble eastward with locally heavy rainfall situated within the confines of the linear segment thanks to focused ascent along the cold front. Meridional flow ahead of the mean trough will pump deeper moisture poleward allowing for 2-3 deviations above normal PWATs to be positioned across the Mid-Atlantic and Northeast with as much as 4 standard deviations forecast for portions of Northern New England. There are two main areas of concern for flash flooding potential, both of which are within the terrain and also have the added effect of snow melt with NOHRSC snow-water equivalent (SWE) estimates pinpointing some modest SWE available to couple with the heavier rainfall. The first area is within the central Appalachians across the high country of WV into Western MD where organized convection out of OH will likely propagate eastward with forcing maximized within the terrain encompassing Tucker/Grant/Randolph counties in WV, and Garrett county in MD. There's a very good signal for at least 1" of rainfall during Wednesday morning with locally as high as 2" within some hi-res deterministic. Given the current SWE available and expected heavy rainfall, some flood concerns will arise as regional streams and rivers could flood their banks in a few locations, especially within the rugged terrain situated in those aforementioned counties. There will be a transition back to snow after the cold front moves through the region and upslope flow takes over, so the threat is mainly in the first half of the forecast cycle, but the threat is certainly warranted. The second area of interest is across Upstate NY into New England where the same issues will be found as locally heavy rains mixed with the rapid snow melt could induce some flooding prospects where SWE is readily available and rainfall rates reaching up to 0.5"/hr prior to the cold front passage. The the main concerns will be during the morning and early afternoon hours Wednesday before the cold front pushes through and the falling height pattern and 1000-500mb thicknesses crash rapidly post front with snow taking over shortly after. There's a bit less enthusiasm for the rainfall totals reaching above 1-1.25" on guidance currently, but the environment is ripe for some totals to reach within that 1.5-2" range which would spell a better risk for flood potential given the components mentioned above. The Marginal Risk in place from the previous forecast was changed very little as the overall consensus of the pattern evolution and expected rainfall is fairly lock steady outside some shifts in the QPF maximums due to convective variability. ....Pacific Northwest... The next atmospheric river will take shape across the PAC Northwest with a minor to bordering moderate AR forecast expected as IVT values between 500-700 kg/ms are expected to advect into WA/OR by Wednesday morning, onward. This event will be characterized by longevity of moderate rainfall and less of short-term magnitude of rainfall as probabilistic fields are all fairly tame for the prospects of any hourly rates reaching the 1" mark. Most of the heaviest rain will settle between 0.5-0.75"/hr with the heaviest rainfall positioned over northwest OR where both the GEFS/ECENS means both pinpoint the heaviest precip totals within the period. Expect 2-4" to be fairly common within the coastal plain extending from the Olympic Peninsula down through southwest OR as we move into Thursday morning. When you add that on top of what is expected during the prior forecast period (12z Tue - 12z Wed), local totals within that above corridor will reach 3-6" over the course of 48 hrs. While this is not a textbook flash flood concern, any poor drainage areas and urbanized locales will be at the highest risk for flooding during the period. The atmospheric river will continue to propagate southward down the Pacific coast with a continued threat heading into the D3 period. The previous MRGL risk was largely unchanged as the synoptic pattern and associated rainfall forecast remains on track. Kleebauer Day 3 Valid 12Z Thu Feb 29 2024 - 12Z Fri Mar 01 2024 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS COASTAL OREGON INTO PORTIONS OF NORTHERN CALIFORNIA... Atmospheric river will continue to push southward with heights falling rapidly across the PAC NW as the mean trough begins pivoting towards the coast. A steady stream of moisture will be positioned from southwest to northeast with flow generally banked against the terrain over northern CA where heavy precip will likely develop Thursday morning and continue through much of the afternoon before waning. Totals of generally 2-4" will be common across the northwest CA coast, inland into the foothills of Mount Shasta down into the northern Sierra. This area has been involved in multiple events over the past month with top soils generally moist with NASA SPoRT soil moisture readings running into the 80th percentile. Will be monitoring the snow levels as the system moves closer to the coast as height falls and surfae cold front will induce a rapid drop of the forecast snow levels as colder air is brought down from the upper levels, shifting the forecast dynamic from rain to snow accumulation. As of now, the best chance for flooding will be along the coastal plain and areas below 6000ft MSL leading to a MRGL risk to encompass the lower elevations from coastal OR down into northern CA. Kleebauer Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!6eyCCy2Y0Qp2-0-mvn-x17ZS8eo24uB77YueXZFmPp5R= BrBV9VdjQ1Vici2_76cSjdK3kfAEKDuNkbfJzQdy7eZHX1k$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!6eyCCy2Y0Qp2-0-mvn-x17ZS8eo24uB77YueXZFmPp5R= BrBV9VdjQ1Vici2_76cSjdK3kfAEKDuNkbfJzQdyoxGcCcQ$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!6eyCCy2Y0Qp2-0-mvn-x17ZS8eo24uB77YueXZFmPp5R= BrBV9VdjQ1Vici2_76cSjdK3kfAEKDuNkbfJzQdyc_ow0Tc$=20 $$ =3D =3D =3D To unsubscribe from WX-STORM and you already have a login, go to https://lists.illinois.edu and use the "Unsubscribe" link. 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