Quantitative Precipitation Forecast Discussion NWS Weather Prediction Center College Park MD 524 AM EST Sat Mar 10 2018 Final Day 1, Day 2 and Day 3 QPF Discussion Valid Mar 10/1200 UTC thru Mar 13/1200 UTC Reference AWIPS Graphics under...Precip Accum - 24hr Day 1... ...Lower MS valley into the TN Valley and Southeast... A heavy rainfall event still appears likely Saturday evening into the overnight period across portions of AR/MS/AL, although still some lingering uncertainties with regards to the exact axis and magnitude of highest totals. Models are however in much better agreement than they have been...and was thus able to add in some more detail to the QPF forecast than our previous issuance. Showers will continue this morning into the afternoon from the central Gulf Coast northeast into the TN valley associated with a weak lead wave...with this initial activity generally being light in nature given limited instability. By around 0z this evening will begin to see better mid/upper level forcing move back into the lower MS valley. In response should see an uptick in 850 mb moisture transport and a tightening of the low level west to east stationary boundary. The combination of this increased synoptic support and low level moisture convergence should result in an expansion of convective activity by this evening across AR/MS/AL along/near the aforementioned boundary. Models all seem to be in agreement with this evolution occurring. PWAT values are only marginally above average...and not really at the magnitude you would like to see for efficient heavy rainfall. Although the values approaching 1.2" would still support the potential for some 1-2" an hour rainfall rates. 850 mb moisture transport is also not as strong as you would normally see for excessively heavy QPF amounts. Instability is not forecast to be that high...although does look we initially see enough surface based instability over AR for development...and enough elevated instability further east to sustain convection into MS/AL. Thus there will be enough increased mid/upper level forcing and instability present to allow for the development of robust convection by this evening. While the magnitude of 850 mb moisture transport is not great (as mentioned above)...the corridor of enhanced transport is broad. This thus supports an elongated axis of moisture convergence...which should be supportive of some west to east training and repeat convection. It is this factor that will likely result in a corridor of 2-3" (locally higher) of rainfall and the development of at least some flash flood threat into tonight (despite the marginal pwats/instability). As mentioned above, models are in better agreement with this convection and thus QPF totals through the day 1 period. Our axis of 1-3" stretches from central AR east into portions of central/northern MS/AL (highest in MS/AL where some training is most likely)...and this is the consensus location amongst the latest global and high res models. As far as the magnitude goes...did lean more towards the high res guidance...which were consistently higher than the global guidance. This peaks amounts in the 2-3" range (although locally could see upwards of 4"). Amounts much higher than this seem unlikely at this point given marginal PWATs/moisture transport and the progressive nature of the system. Overall our axis of higher totals is pretty similar to our previous forecast. Although this forecast does cut back quite a bit on the extent of the 1" area. Some of the previously broader models with the QPF have since backed off...and it is becoming clearer that the heavier totals should stay confined to a narrower axis near the low level convergence axis. ...South Florida... The presence of a mid level wave and the development of impressive upper level divergence will result in widespread convective development over the Gulf of Mexico today. This activity will move east towards central and southern Florida through the day. In general think the activity will be in a weakening state as it moves ashore given limited instability. However appears increasingly likely that at least some steady showers...and possibly a few embedded heavier convective cores will move across southern Florida this afternoon and evening. Will need to monitor trends through the day...as some signs in a few CAMs that some heavier convection could impact far southern Florida and the Keys by this evening as some instability tries to work its way north. WPC followed a multi model blend for QPF across this region...which is a decent increase from our previous forecast. For now kept areal averaged amounts below 1"...but as mentioned above will need to monitor trends as some chance we see locally higher totals. ...Southwest... A wave moving into the southwest will bring a broad area of precipitation into southern CA and inland into the remainder of the southwest through the day 1 period. An impressive plume of moisture is noted...with PWAT values pretty much as high as you can get this time of year over the southwest. This combined with the wave/upper jet certainly supports an expansive area of light to moderate precipitation mounts. Limiting factors will be the flatness of the wave...which is not really supportive of heaver rates...and also low level wind fields that are not all that anomalous. So while we certainly will see some orographic enhancement over southern CA...not expecting rates to be that impressive. Current indications are for rainfall rates to generally peak within the 0.25"-0.35" range within an hour across portions of the southern CA terrain this afternoon and evening. Models have trended further south with the wave and quicker. This results in a shift south in the heavier QPF across CA and a further inland extent of precipitation across NV/AZ/NM into tonight. Expecting generally between 0.5"-1" across much of coastal southern CA...with 1-3" possible in the more favored terrain. Days 2/3... ...Four Corners... A shortwave trough shifts east from southern California Sunday morning and weakens as it runs into the western CONUS trough over Arizona/New Mexico. Precipitable water around 1 inch (+4 standard deviations for southern AZ) Sunday morning, decreasing thereafter with a quarter to half inch QPF over the higher elevations of northern AZ/NM before (unfortunately the dry TX panhandle) being squashed by high pressure over the plains. ...Mid/Lower MS Valley across Southeast and Mid-Atlantic... 00Z Guidance has much better consistency than previous model suites for the developing low over the southeast Sunday night. A northern stream shortwave trough currently over MT dips southeast to the mid-South through Sunday evening before shifting offshore from the Carolinas late Monday. A second trough drops from Ontario Sunday night. The 00Z UKMET draws this trough into the Great Lakes region quicker and allows phasing with the trough over the Mid-South and thus takes a farther west track. Eastern Gulf and Gulf Stream moisture flows into this system on 20 to 30 kt 850mb flow. The progressive nature of the low and its associated cold front should limit excessive rainfall risk over the southeast. WPC QPF was based primarily on the 00Z GFS/ECMWF and the 00Z GEFS. ...Northeast... 00Z guidance consensus is for the developed low pressure system to track north off the New England coast Monday night through Tuesday. The 00Z GFS/ECMWF are well offshore with only southeastern New England/MA getting notable precip from the outer comma head precip shield. The 00Z UKMET however is notably farther west with its track producing another Nor'easter for the northeast. The last two UKMET runs have featured this track while other guidance has wavered, but has not been this far west. Both the 00Z GEFS and ECMWF ensemble means feature greater spread west. WPC QPF was based on a 40/40 00Z GFS/ECMWF, 10 percent 00Z UKMET, and 10 percent GEFS ensemble mean. ...West Coast... A deep longwave trough digs off the west coast Sunday night through Monday. Associated precip reaches the west coast Monday morning. One inch PW riding a 40kt southerly jet provides enhanced rainfall for the coast ranges north from northern CA. Generally up to three-quarters of an inch QPF for the northern CA coast ranges. Chenard/Jackson Graphics available at www.wpc.ncep.noaa.gov/qpf/qpf2.shtml