Quantitative Precipitation Forecast Discussion NWS Weather Prediction Center College Park MD 626 PM EDT Thu May 24 2018 Final Day 1, Day 2 and Day 3 QPF Discussion Valid May 25/0000 UTC thru May 28/0000 UTC Reference AWIPS Graphics under...Precip Accum - 24hr Day 1 ...Northern California---central to southern Oregon---northwest Nevada... Overall very good agreement on the slow east northeast movement of the developing closed low off the central California coast--with this system pushing inland Friday into central to northern California. Strongly difluent upper flow to the north and northeast of this closed low will support increasing convection Friday from northern California---across northwest NV and into southern to central Oregon. Much above average pw values in this strongly upper difluent pattern will support heavy to isolated excessive rainfall amounts across these areas. No changes made to the marginal risk on the Excessive Rainfall Outlook. ...Upper Great Lakes---Upper MS Valley---portions of the Great Plains... A fairly large region of scattered convection possible day 1 ahead of surface cold front expected to move slowly eastward across the Great Plains. Continued low confidence in this scattered convection regime---with a large spread with respect to the details of potential max qpf areas. There is some model consensus for a potential max region from central to eastern KS---northeast OK into southwest MO and northwest AR. Another possible max region is over portions of the Upper MS Valley into the Upper Lakes in a region of max isentropic lift along and to the northeast of a northeast moving warm front. With pw values remaining above average across these areas---localized areas of heavy precip possible where convection does maximize. ...Central Gulf coast into the Southeast... Not a lot of changes expected to the recent pattern across the Central Gulf coastal region and into the Southeast day 1. PW values expected to remain 2+ standard deviations above the mean along and to the south of the weakening stationary front draped from the Lower MS Valley into the Southeast. A nearly stationary mid to upper level vort over central Georgia will continue to act as a focus for heavy precip totals in the vicinity of this feature over large portions of Georgia. Somewhat higher confidence with heavy precip totals near this feature---with lower confidence elsewhere across the Central Gulf coast and Southeast. There will continue to be the potential for slow movement of cells and heavy to locally excessive rainfall amounts across these areas day 1. ...Keys into South Florida... Precipitation should be increasing day 1 across the Keys and South Florida as the developing area of low pressure in the vicinity of the Yucatan Peninsula begins to push northward. With tropical moisture being advected northward on the eastern side of this low---the heavy precip threat will increase day 1---continuing into the day 2 and 3 time period. See the latest NHC outlooks or potential future advisories for additional information on this system. ...Northern Maine... Height falls dropping southeast across central QB will support the potential for several waves of precip to streak in a west northwest to east southeasterly direction across northern Maine this period. The potential for several waves of precip will support moderate to locally heavy totals---with the latest models showing fairly good agreement here. Days 2/3... ...Southeast/eastern to central Gulf Coast/FL... Deep tropical moisture within confluent flow continues to combine with diurnal heating/destabilization to produce widespread showers/storms from southern SC to central AL along a weakening surface front under a remnant 850 mb circulation over GA. Above normal moisture leads to a continuation of a Slight Risk for excessive rain over GA for Day 2 (12Z Fri-12Z Sat). Guidance continue to depict persistent, deep flow of moisture from the Bahamas into southeast FL with peak precipitable water values of 2 to 2.25 inches so showery conditions are expected along the southeast coast. The pattern favors inland penetration of the sea breeze from the east coast so peak afternoon convection should occur over interior portions of the FL peninsula to the west coast. Lapse rates are not as high as usual due to warm mid level temps, so bands of larger scale lift are needed to trigger FL convection. Convective activity into southern FL warrants a Marginal risk for excessive rainfall there for both Days 2 and 3 given recent wet conditions. Trends on the advection of the tropical moisture and embedded convective activity continue with the GFS trending west and the ECMWF still trending slower. A blend of the 12Z GFS/ECMWF was used for QPF. ...Mid-Atlantic... Upper level ridging over the eastern CONUS will allow tropical moisture to continue to lift north from the southeast, reaching the Mid-Atlantic Friday night into Saturday. A cold front will shift south across the northeast. This simple low level convergence of much above normal moisture (reaches 2.5 standard deviation over the central Mid-Atlantic on Sunday) may cause a predecessor rainfall event over a portion of the Mid-Atlantic late this weekend into next week. The 12Z ECMWF greatly increased the QPF east of NJ Saturday night into Sunday. Signals like this will need to continue to be monitored for what is a difficult to pinpoint phenomena (predecessor rainfall events). ...Pacific Northwest/Great Basin/Northern Rockies... A closed mid-upper level low drifts onshore and from CA to UT Friday night through Sunday. Southerly flow ahead of the circulation maintains above normal moisture. Continued the Marginal risk for excessive rainfall over OR/far northern CA for Day 2 (12Z Fri-12Z Sat) where 0.75 to 1 inch PW persists with low to mid-level convergence. Saturday the 700 mb circulation moves northeast out of NV towards the WY/UT border. Continuing 700 mb convergence along and northwest of the low track promotes showers across northeast NV across southern ID and much of MT. With low pressure developing on the northern Rockies, low level convergence leads to showers developing over the Rockies into the northern high Plains Saturday night. QPF was based on the 12Z GFS/ECMWF. ...Great Lakes... Activity Friday night into Saturday will focus ahead of a low level front and prefrontal moisture plume with low-mid level convergence on the nose of the low level jet over mid-upper MS Valley/northern Great Lakes. Locally excessive rainfall is likely again with the plume of sub-tropical moisture extending across this area into Canada. No excessive risk was raised at this time, though that may need to change for Day 1. QPF was based on the 12Z GFS/ECMWF. Oravec/Jackson Graphics available at www.wpc.ncep.noaa.gov/qpf/qpf2.shtml