Quantitative Precipitation Forecast Discussion NWS Weather Prediction Center College Park MD 556 AM EDT Sun Mar 11 2018 Final Day 1, Day 2 and Day 3 QPF Discussion Valid Mar 11/1200 UTC thru Mar 14/1200 UTC Reference AWIPS Graphics under...Precip Accum - 24hr Day 1... ...Lower MS Valley into the Southeast ... As of 06z this morning convection stretches across portions of AR/MS/AL forming within an environment of low level moisture convergence and steep mid level lapse rates. May continue to see some training of this convection through 12z across portions of MS/AL. Thereafter most indications amongst the CAM guidance is for convection to become more progressive in nature off to the southeast. This seems reasonable...as by this time we begin to see the stronger wave currently over the central plains move east and begin to interact with the lead wave...resulting in a more progressive push and more of a southwest to northeast orientation of the 850 mb moisture convergence axis (less conducive to training than the west to east axis we are seeing early this morning). Convection is however expected to maintain its intensity a bit more than previously forecast as it moves south towards the Gulf Coast...and expect the convection currently initiating over northeast TX to expand in coverage into the early morning hours...resulting in a nearly continuous southeast to northeast axis of convection stretching from LA into southern GA. None of the 0z high res CAMs were doing a particularly great job...however thought the more progressive ARW and ARW2 were handling the ongoing activity better than the others. The 0z UKMET was also not all that bad. Thus the WPC QPF follows closest to these models...with extrapolation of the current activity also used at the beginning of the period. This results in an increase in amounts over our previous forecast...with an axis of 0.75"-1.5" across a good portion of the lower MS Valley into the southeast. ...Mid MS Valley into the OH/TN Valley and western Mid Atlantic... The well defined shortwave over the central plains will move into the MS Valley today. Models are trending towards a more well defined wave and area of forcing with this feature as it moves from the mid MS Valley into the Oh/TN Valley and the western Mid Atlantic through the day 1 period. This combined with some dual left exit and right entrance upper jet dynamics...should support a more focused area of lift and even a surface low reflection ahead of the wave. Given this trend, it is not surprising that model QPFs are also trending wetter across this area into tonight. Eventually the better energy will shift offshore to the developing coastal low...but at least through 12z Monday should see some good QPF ahead of this mid level wave. WPC increased QPF over our previous forecast...using a consensus of the 0z GFS/UKMET and the 0z high res CAM guidance. Portions of SC/NC will generally be in between the convective precip over the south and the precip associated with the mid level wave to the northwest. Thus QPF amounts here were lowered from our previous forecast. Again WPC utilized a blend of the GFS/UKMET and the 0z high res models. ...Southwest... precipitation associated with the southwest wave will continue moving east today into AZ/NM. The 0z models were in very good agreement with this feature and resultant QPF...thus WPC utilized a multi model blend. There was a unanimous slowing trend noted...which lowers QPF amounts central and eastern NM. Days 2/3... ...The West... Deep low pressure approaches the West Coast Monday and reaches the shore Tuesday. Locally heavy stratiform rain reaches the northern CA coast Monday night as a plume of 0.75 inch PW is brought in on a 50kt low level south flow. A shortwave trough rounding the low shifts the focus for heavy precipitation inland across northern CA with highest rates at higher elevations. Height falls associated with the low will allow snow elevations to lower. Therefore...no excessive rainfall outlook is expected at this time. Expect areal averages of 1 to 2 inches liquid equivalent precip for the CA Cascades and northern Sierra Nevada for Tuesday. The precip shield spreads inland and north through this time with lesser rates downwind of the barriers of the CA coast ranges and Sierra. There was generally good model agreement with this low pressure system, so the WPC QPF was based on a blend of the 00Z ECMWF/GFS. ...Carolinas to New England... A northern stream trough shifting southeast across the mid MS Valley becomes neutrally tilted and closed as a low at 500mb by the time it reaches the TN Valley Monday morning. This developing low will track east across the Carolinas Monday before turning north up the Mid-Atlantic coast Monday night as it begins to interact with an upper low dropping south across the Great Lakes. Phasing of these two lows allows rapid intensification of the surface low along the coast through Tuesday. Uncertainty still remains with the track of the surface low, but the 00Z model suite favored a return back to a near coast low with the precip core spreading farther inland. WPC QPF was based on a 70 percent blend of the 00Z GFS/ECMWF operational runs and 30 percent blend of the 00Z GEFS/12Z ECMWF ensemble means. QPF from the GFS and ECMWF ensemble means is greater inland than the operational runs. Banded precip from the developing low will allow locally intense precip rates on the north and west sides of the low and should be ongoing Monday morning over the southern Mid-Atlantic. Further info on snowfall can be found in the QPFHSD discussion. The upper low lingers over the Great Lakes and maintains a trough to the surface low that is expected to track to the north of Maine Tuesday night. This enhances lake effects on the wrap around of the storm. ...Florida... The cold front associated with the developing low crossing the Carolinas Monday morning shifts south across Florida Monday. 1.5 inch PW ahead of this front will allow locally heavy rainfall. Areal average QPF are generally a quarter to half inch with this progressive front, so no excessive rain risk is expected. Chenard/Jackson Graphics available at www.wpc.ncep.noaa.gov/qpf/qpf2.shtml