Area Forecast Discussion
Issued by NWS St. Louis, MO
Issued by NWS St. Louis, MO
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250 FXUS63 KLSX 090849 AFDLSX Area Forecast Discussion National Weather Service Saint Louis MO 349 AM CDT Tue Jul 9 2024 .KEY MESSAGES... - Widespread rainfall associated with Beryl`s remnants is forecast to overspread most of the area this morning and continue through the afternoon/evening. Additional rainfall amounts of 1-3" with locally higher amounts are forecast along and south/east of the I-44/I-55 corridors in Missouri and Illinois respectively. - The main impacts from this event include river flooding (most likely on the Meramec, Big, Bourbeuse and Black River basins), isolated flash flooding, gusty winds (30-40+ mph at their peak), and a low (but non-zero) threat for a tornado or two in parts of southeast Missouri and southern Illinois. - Generally quieter weather is forecast Wednesday-Saturday with temperatures warming back to near normal and chances for showers and thunderstorms confined to Wednesday/Thursday afternoons. && .SHORT TERM... (Through Late Tonight) Issued at 341 AM CDT Tue Jul 9 2024 The remnants of Tropical Cyclone Beryl are currently located in far southwest Arkansas as of 0800 UTC. The curve more to a northeast heading has begun, with the surface low expected to move into southeast Missouri (near KPOF) around midday. Ahead of the surface low, widespread light to moderate rain is spreading northward out of southeast Missouri and southwest Illinois. This area of rainfall is coming in response to differential positive vorticity advection downstream of the midlevel trough, strong upper-level divergence beneath the right-entrance region of a jet streak over southern Iowa, and strengthening low-level moisture convergence. The divergence centered around 300 hPa is particularly strong/impressive, with even hints of some jet coupling. The axis of heaviest rainfall through early tonight should fall along the area of very strong low-level moisture convergence. This axis is forecast to setup near the I-44/I-55 corridors in Missouri and Illinois respectively. Rainfall rates in these locations are likely (60-80%) to climb above 0.25"/hr for approximately 6 hours at any given location per the latest HREF guidance. Peak hourly rates of >0.50"/hr also may occur, but probabilities are lower (10-30%), which suggests these rates may be more isolated and transient in nature. By the time the rain pulls away from west to east late this afternoon into the early overnight hours, widespread 1-3" of rain is forecast across parts of east central and southeast Missouri as well as southwest and south-central Illinois. Locally higher amounts (up to 4") are possible. Heavy rainfall is just one impact that is expected with this storm system through tonight, with additional impacts detailed below: RIVER FLOODING: Based on the forecast rainfall totals through tonight, (new) river flooding is expected along at least portions of the Meramec, Big, Bourbeuse, and Black River basins. Chances of moderate flooding within these basins at this time look low, but if rainfall totals are a bit higher than forecast and line up along a basin, it still could occur. As for the ongoing flooding on the Missouri and Mississippi River mainstems (and backwater points from the Mississippi River), the rainfall axis is very likely to stay far enough southeast to only have minor impacts. More specifically, this rain is expected to largely slow down the fall on the Missouri River closer to the confluence and most of the Mississippi River. The exception on the Mississippi River is Chester, which is far enough south to feel more of the impacts of this rainfall. Chester is forecast to crest (34.0 feet) about 1.5 feet higher than a couple of days ago on 7/12. A crest above moderate flood stage (35.0 feet) is possible if heavier rain ends up falling. The latest HEFS shows about a 25% probability for moderate flooding. FLASH FLOODING: The flash flood threat looks to be isolated given the steady, mostly light to moderate hourly rainfall rates expected today. HREF probabilities detailed above for hourly rainfall rates suggest less than 0.50"/hr. Three-hour flash-flood guidance is generally near 2", with 1-hr guidance around 1.50". Areas of (greater) concern are in those locations which have seen localized very heavy rainfall and flash flooding over the past 24 hours, and in any locations that manage to see convection later today. The best chances of convection and higher rainfall rates appear to be in parts of southeast Missouri and southwest Illinois this afternoon. GUSTY WINDS: Wind speeds and gusts are expected to increase this afternoon through the early evening as the surface low moves through southeast Missouri into east/southeast Illinois. Model soundings show quite a bit of variability with magnitude, but all suggest strong (30-40+ knots) northeasterly flow at 925 hPa. Gusts of 30-40+ mph are forecast to move from metropolitan St. Louis eastward, roughly from 6PM through 1AM tonight. HREF probabilities for 35+ mph gusts peak at about 50-70% but drop off to 10-20% for advisory-criteria gusts (45+ mph). I cannot rule out some isolated gusts near advisory criteria, but I am not confident in this being widespread enough (if any happen at all) to issue an advisory. TORNADOES: A tornado or two is possible this afternoon for a portion of southeast Missouri and southern Illinois. There is greater uncertainty in our area, given the likely track just to the southeast of the area (hence higher tornado probabilities per the Storm Prediction Center closer to the Ohio River). Most CAMs keep upright convection/mini supercells just to our southeast, but it may be a close call. The low-level shear/helicity is unsurprisingly quite strong near the track of the surface low later today. Temperatures today will be quite chilly for the middle of July. The triumvirate of rain falling basically all day, northeast surface winds, and low ceilings should lead to very little in the way of warming from this morning`s lows (maybe 2-3F in most locations). Highs in the 70s are forecast, but readings topping out near 70 degrees are expected within the heavier rainfall axis in parts of east central and southeast Missouri. Those temperatures would be 15 to near 20 degrees below normal for the date. We will also have to keep an eye on the potential for fog development late tonight toward early Wednesday morning in parts of central Missouri. Sky conditions are expected to clear out behind Beryl`s remnants. Combined with lightening winds, this could be a good recipe for fog development. That area also should get at least some additional rainfall today, so there should be plenty of residual near-surface moisture. Guidance at this time only has a weak signal for widespread fog, possibly due to winds potentially staying up just enough. Gosselin && .LONG TERM... (Wednesday through Monday) Issued at 341 AM CDT Tue Jul 9 2024 (Wednesday - Thursday Night) A slow-moving midlevel shortwave trough is forecast to move through the Upper Midwest into the Great Lakes during the middle of the week. The mid-Mississippi Valley will be on the southern periphery of the better mid/upper level support associated with this feature, but there should be enough for isolated-scattered convective development each day, mainly during peak heating (late afternoon) hours. Temperatures are expected to warm up from the chilly conditions of today, largely in the mid 80s on Wednesday and back closer to normal on Thursday (upper 80s to near 90 degrees). (Friday - Monday) Ensemble means from the GEFS/EPS continue to suggest our area will get more and more influence from the mid/upper level ridge over the intermountain west. It is forecast to shift eastward and become more broad. This pattern is expected to yield largely dry conditions along with a return to above normal temperatures Sunday into Monday. Forecast uncertainty increases a bit with respect to a possible frontal passage sometime Sunday-Monday. Questions include the timing of the front and whether it will actually move through the area. If not, well-above normal temperatures (mid 90s to near 100 degrees) could begin on Monday and continue later on into next week. Gosselin && .AVIATION... (For the 06z TAFs through 06z Tuesday Night) Issued at 1041 PM CDT Mon Jul 8 2024 The primary concern during the 06Z TAF cycle will be the potential for impacts from the remnants of tropical depression Beryl. At the start of the period, some lingering showers may impact local terminals briefly, particularly at UIN. However, this is not expected to be widespread or to last long. Tomorrow morning, ceilings are expected to lower to MVFR levels as more widespread rain arrives. This rain is expected to steadily intensify through the day, and all terminals are likely to be impacted to varying degrees. St. Louis area terminals are expected to see the most prolonged and significant impacts, with IFR ceilings possible at times, and even potentially IFR visibilities within stronger convection. Occasional lightning can`t be ruled out, but is not expected to be frequent. Rain rates will steadily diminish during the evening tomorrow, and ceilings/visibilities are expected to improve as well. BRC && .LSX WATCHES/WARNINGS/ADVISORIES... MO...Flood Watch through late tonight for Jefferson MO-Lincoln MO- Saint Charles MO-Saint Louis City MO-Saint Louis MO-Sainte Genevieve MO. Flood Watch through this evening for Audrain MO-Boone MO- Callaway MO-Cole MO-Crawford MO-Franklin MO-Gasconade MO- Iron MO-Madison MO-Moniteau MO-Montgomery MO-Osage MO- Reynolds MO-Saint Francois MO-Warren MO-Washington MO. IL...Flood Watch through late tonight for Bond IL-Calhoun IL-Clinton IL-Fayette IL-Greene IL-Jersey IL-Macoupin IL-Madison IL- Marion IL-Monroe IL-Montgomery IL-Randolph IL-Saint Clair IL-Washington IL. && $$ WFO LSX