638 FOUS30 KWBC 221956 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 355 PM EDT Fri Mar 22 2019 Day 1 Valid 16Z Fri Mar 22 2019 - 12Z Sat Mar 23 2019 ...A MARGINAL RISK OF EXCESSIVE RAINFALL EXISTS NEAR THE NEBRASKA/WYOMING BORDER AND THE TEXAS PANHANDLE INTO WESTERN OKLAHOMA... 1530z Update: Have decided to add a Marginal Risk across portions of the TX panhandle into western OK. Additional convection will initiate across eastern NM this afternoon...and will initially be progressive off to the east into the TX panhandle. Some signal that we could see some backbuilding of activity this evening into the first half of the night...resulting in some flash flood potential. Will see an increasing axis of 850mb moisture transport towards sunset directed into TX Panhandle...with this axis remaining nearly stationary for at least half the night...before the wave from the west helps push things eastward. Given the axis and orientation of this moisture transport axis...could be favorable conditions for some backbuilding of cells to the south southwest into the low level jet and instability gradient. Not anticipating a high end threat...but this setup could produce some pockets of 2-4" of rain in a couple hour window...which may begin to cause some concerns across more sensitive areas. Scattered convection is also probable further south along the dryline over west TX this afternoon. Any supercells that form here could become slow moving or briefly anchor over one location. This could cause an isolated flash flood threat here as well...but the coverage of this threat appears too low to warrant any Risk area at this time. Chenard ...previous Discussion... The combination of recent snow melt and moderate to heavy rain could lead to issues across this region. Hourly rain totals up to 0.5" are possible as precipitable water values climb towards 0.75" in the central High Plains to the north of a cyclone moving through southeast CO and MU CAPE values rise close to 500 J/kg. The period when heaviest rain is expected in the 2200-0600 UTC window per the 06z HREF probabilities of 0.25" and 0.5" an hour. Because of the recent snow melt, flash flood guidance values are quite low, 1-1.5" in three hours and 1.5-2" in six hours. Local spots could exceed the six hour guidance in what is expected to be a longer duration heavy rain event. Roth Day 2 Valid 12Z Sat Mar 23 2019 - 12Z Sun Mar 24 2019 ...THERE IS A MARGINAL RISK OF FLASH FLOODING OVER PORTIONS OF THE LOWER MISSOURI VALLEY... ...Eastern Nebraska, northeast Kansas, northwest Missouri and southwest Iowa... The mid-upper level closed low currently over the Great Basin will push eastward across the Plains states during this period. As the associated lee-side surface low strengthens and moves slowly eastward, a relatively strong (30-40 knots at 850mb) southerly flow of Gulf of Mexico moisture will stream northward across the southern and central Plains during Saturday. Isentropic lifting over a pair of surface frontal zones (a warm front extending from the low and a weakening stationary front to its north) will support showers developing across the risk area on Saturday. There may be a hiatus in the rainfall Saturday evening before convection breaks out with increased instability in the vicinity of the upper low. Overall, precipitation amounts don't look particularly heavy with this system. Precipitable water values generally range from .75 to 1.00 inch, which equates to only about 1-1.5 standard deviations above normal. And most unstable CAPE is forecast to at best be around 200-400 J/kg. That said, the ground over which this rain is forecast to fall is quite saturated, and while the instability and moisture are marginal, lifting will be enhanced by a highly diffluent flow at the upper levels. As a result, isolated heavier precipitation rates are possible, especially Saturday evening when instability will be strongest. The Marginal Risk area was extended a little farther west in Nebraska from the previous issuance to account for the elevated potential for convection under the upper low center. ...Eastern Oklahoma and northeast Texas... Along and ahead of the cold front trailing the low moving through the Plains, instability and precipitable water values will be higher. Some of the models are showing a signal for higher QPF across eastern OK, western AR and northeastern TX, with maximum amounts of 2-3+ inches. The latest guidance, however, continues to show quite a lot of spread, thus confidence in the placement of any higher QPF amounts is low. Also, much of this region has had below normal precipitation over the past 2-4 weeks, so even with isolated heavy precipitation, the flooding potential is relatively low. Klein Day 3 Valid 12Z Sun Mar 24 2019 - 12Z Mon Mar 25 2019 The probability of rainfall exceeding flash flooding guidance is less than 5 percent. Klein Day 1 threat area: https://www.wpc.ncep.noaa.gov/qpf/94epoints.txt Day 2 threat area: https://www.wpc.ncep.noaa.gov/qpf/98epoints.txt Day 3 threat area: https://www.wpc.ncep.noaa.gov/qpf/99epoints.txt $$