Quantitative Precipitation Forecast Discussion NWS Weather Prediction Center College Park MD 513 AM EDT Wed May 02 2018 Prelim Day 1, Day 2 and Day 3 QPF Discussion Valid May 02/1200 UTC thru May 05/1200 UTC Reference AWIPS Graphics under...Precip Accum - 24hr Day 1 ...Central Plains into the Mid/Upper MS Valley and Great Lakes... Two discrete clusters of convection were prevalent early this morning per the mosaic radar loops. One was draped across the upper Midwest within an area of favorable dynamics south of the entrance region of an upper level jet streak, ahead of an old MCV, and along/se of the quasi-stationary surface front. Elevated instability across this region is modest -- with mucapes around 1000 j/kg supporting pockets of 1+ inch/hour rainfall rates. The other area is farther south across portions of the lower MO Valley from northeast KS into northern MO -- where the convection has effectively grown upscale into an MCS overnight. Over this region short-term rainfall rates were more robust (pockets of 1.5-2.0"/hr), owing to the stronger deep-layer instability (mucapes closer to 2000 j/kg), strong low-level moist inflow (50+ kts), and favorable alignment of the low-level jet (llj) to the mean wind in favoring upwind propagation and cell training. The high-res guidance (including the more recent HRRR and experimental HRRR runs) continue to show a diminishing trend with the MCS later this am with the dissipation of the llj. However, with a new quasi w-e outflow boundary laid down s-se of the main frontal boundary, the concern is that the new convection firing later in the day with the peak heating -- and overnight as the activity once again grows upscale into an MCS -- will again be displaced farther south of the main frontal boundary. Thus not surprisingly the 00Z model suite continues to show quite a bit of discrepancy with the QPF details from late Wed into early Thu morning. There is consensus among the guidance with the axis of heavier rainfall farther north, i.e. from IA to northern IL-IN and southern MI, associated along and south of the quasi-stationary surface front and closer to the aforementioned upper level jet streak (enhanced right entrance region forcing). However, outside of the 00z ECMWF and UKMET across south-central and southeast KS, the global guidance continues to struggle with the likely MCS evolution Wed night-early Thu farther south along the aforementioned outflow boundaries and toward the (weakening) mid-level cap -- i.e. across southern MO and potentially into northeast OK-far northern AR). WPC opted to some extent incorporate the ECMWF and UKMET idea with the QPF across KS -- however lean a bit toward the WRF-ARW and NMMB blend from 00-12Z Thu across southern MO. As is often the case with more convectively-aided dynamical forcing in the warm season, well south of the main surface front with deep-layer ridging in place, the question becomes "how far south" will the convection align. In such cases it seems that the ARW seems to do a good job leading the way with the southern trend. ...Central TX... The models, particularly the high-res CAMs, again show a cluster of heavier QPF east of the Big Bend initiating along the dry line late in the afternoon and growing upscale (somewhat) overnight Tue with the uptick in low-level inflow. The global models in fact all show a small MCV developing as a result. WPC noted a small pocket with areal-average totals between 0.50-1.0"; however, most of the high-res CAMs show isolated max amounts closer to 1.5-2.0". The strong low-mid layer shear (0-6 km bulk shear over 50 kts) would favor more forward or downwind propagation than otherwise, which would limit the threat for training convection. However, well-organized multi-cellular clusters (supercells especially) would favor isolated excessive short term rainfall rates. As a result, WPC will continue with the MARGINAL risk in the new day 1 ERO. Days 2/3... ...Western United States... Only light precipitation should be lingering along the Rockies Front Range in Colorado and Wyoming as the Day 2 forecast begins. Precipitation will then be tapering off and coming to an end during the day on Thursday with less than a quarter of an inch additional precipitation. ...Rooftop of Maine...Early on Day 2... A weak shortwave embedded within well established zonal flow over Southeast Canada will brush the Rooftop of Maine between 00-06z on Thursday. Sufficient deep moisture transport and mild convergence ahead of the wave (near weak surface low along frontal boundary) will support modest QPF values in the .5" range. A blend of the GFS and ECMWF were used to place the axis of the QPF. ...Central Plains to southern Great Lakes to Northeast... At the start of the Day 2 period...12Z Thurs...strong moist return flow out of the western Gulf of Mexico surges north ahead of the emerging mid-level large scale trof out of the Western US with well established dry line from the Big Bend Region to W OK/TX panhandle to the Developing surface low in N Central KS. A trough emerging from the Rockies Front Range early on day 2 will be supporting generally light upslope precipitation north of the main vort path. The main action will be ahead of the trough where the strong mid and upper level dynamics will be interacting with the abundant moisture streaming northward (as mentioned above). There were growing signals on Day 2 that one stripe of heavy raifall will be along and north of a quasi stationary front extending from the Plains towards the southern Great Lakes while there looks to be a second stripe of heavy rainfall extending from the Upper Midwest into the western Great Lakes which extends beyond the international border. It appears that the models were keying in on the moist flow interacting with the baroclinicity of the boundary in the southern area and the strong difluence aloft and interaction with shortwave energy in Canada approaching from the northwest to support the second area. The precipitable water values part of Wisconsin and Illinois are forecast by the models to increase to 1.4 inches or so which is well above normal so suspect that the QPF picture (and associated excessive rainfall) can be altered significantly depending on exactly where and when the convection develops. The marginal risk area was shifted northward in the latest Day 2 Excessive Rainfall Outlook given the consistent shift seen in the numerical guidance. The areal coverage is large since it attempts to encompass the two areas of heavy rainfall as shown by two slight risk areas. No attempt was made to parse the marginal risk areas into two smaller areas at this point given the uncertainty how some very key forcing elements will evolve in the next 48 to 60 hours in this part of the country. Thinking is that the potential for flooding problems will be over the eastern parts of Wisconsin and northeast Illinois where flash flood guidance is lower that in other parts of those states. Eventually, the wave and moderate rains will cross the Northeast by Friday with QPF generally around .5" at the eastern portions of New England to 1" further upstream in eastern Great Lakes...generally supported by a multi-model blend. We tended to favor the latest GFS/ECMWF combination early on which then morphs more heavily towards the ECMWF. ...Southern Plains... Height falls swinging around the southern side of the above mentioned trough, and southern end of a 120 to 140 knot upper level jet will be able to support thunderstorms from parts of Texas into Oklahoma on Day 2 and Day 3. In time, a cold front associated with the systm over the northeast U.S. will push into the region. Thinking is that the convection and the potential for heaviest rainfall will occur on Day 3 over west TX once the front comes into play. Thought the potential was still there on day 2 for a marginal and potential worth a slight risk on day 3. ...South Florida... A trough moving northward across the Bahamas will start nudging showers ever closer to the southeast Florida coastline by early Saturday. WPC manual graphics favored the slower side of the guidance in terms of QPF due to the presence of the subtropical ridge over the southeastern U.S.. Hurley/Bann Graphics available at www.wpc.ncep.noaa.gov/qpf/qpf2.shtml