Quantitative Precipitation Forecast Discussion NWS Weather Prediction Center College Park MD 247 AM EDT Thu Mar 29 2018 Prelim Day 1, Day 2 and Day 3 QPF Discussion Valid Mar 29/1200 UTC thru Apr 1/1200 UTC Reference AWIPS Graphics under...Precip Accum - 24hr Day 1 Central Gulf Coast The well defined southern stream trof moving across the Southern Plains early Thursday morning will be accelerating northeastward and becoming less defined with time day 1 as it pushes across the Lower MS Valley---TN Valley southern Appalachians and into the Mid Atlantic. The organized convection currently producing very heavy rains over east TX into LA is expected to weaken prior to the beginning of the new day 1 time period but still support a max precip area over the central gulf coastal region from southern LA eastward into the Florida panhandle. This activity is expected to be more progressive than current upstream activity---supporting areal average precip totals in the 1-2" range. The expected increased progression should lessen the threat of runoff issues along the central gulf coast compared to ongoing activity--however can not rule out at least a slight risk potential with any organized convection---especially affecting urban areas. OH Valley into the eastern Great Lakes Developing surface low pressure over the OH Valley Thursday will support an increasing comma head/deformation precipitation area over the OH Valley---pushing northeastward into the eastern Great Lakes Thursday evening and across northern NY state into northern New England Thursday night/early Friday. There is a lot of model run to run variations with respect to the qpf axis details across these regions and model to model differences leading to lower confidence. WPC qpf leaned toward the in house qpf mean to mitigate these fluctuations and differences---depicting moderate to heavy precip amounts in the .50-1"+ range across these regions. Northern Rockies into the Northern Plains Northwest mid to upper level flow with embedded shortwaves will persist day 1 from the Northern Rockies into the Northern Plains. Low confidence with respect to the areas of precipitation to the east of the Rockies into the Northern Plains given the model spread. However---model do agree that any precip will be on the light to locally moderate side given pw values that are near seasonal norms but still relatively low. Pacific Northwest/southwest Canada Northeast Pacific shortwave energy moving southeast across central to southern B.C. will help strengthen onshore flow into coastal WA late Thursday/early Friday. The heaviest precip totals likely to the north of the U.S./Canadian border across Vancouver Island and southwest B.C.---with .25"+ totals affecting the northern Washington Cascades and Olympic range. Days 2 & 3 ***East Coast region*** A narrow plume of deep moisture is expected to be situated over the southeast U.S. coast and northern Florida at the beginning of Day 2 Friday morning ahead of a cold front. This area will be under the right entrance region of a 130+ knot 250mb jet, and with advection of 1.5" PWs ahead of the boundary, a narrow band of heavy showers and thunderstorms is expected Friday morning. Models agree that this axis of heavy rainfall will be offshore by Friday afternoon, and this will generally keep rainfall totals under an inch for this region. Lighter amounts are expected to the north across the Mid-Atlantic region and the northeast U.S. where forcing for ascent will be less. ***Dakotas to the Great Lakes*** A series of shortwave troughs will pivot around the southern periphery of a large polar vortex centered over the Hudson Bay region through this weekend. The most potent of these shortwaves will track southeast across British Columbia on Friday and then across the northern plains by Saturday morning. Surface low pressure develops along a surface trough across the Dakotas Friday evening, and then reaching the Great Lakes by Saturday. This region should also be under the favorable left exit region of the northern stream upper jet, and combined with low-mid level frontogenetical forcing, a band of steady precipitation is expected from North Dakota to northern Michigan, with most of this falling as snow. The WPC forecast was based primarily from the GFS/NAM/biased corrected model and closer to previous continuity. The past two runs of the ECMWF appear to be a little too far north with the QPF axis across North Dakota and northern Minnesota. The winter weather discussion (QPFHSD) has additional information regarding the snowfall expected with this. ***Washington to the northern Rockies*** With a series of small perturbations in the upper level flow over the northwest U.S. in the wake of the northern plains winter storm, there should be enough ascent in the mid levels from right entrance jet dynamics to generate mainly light QPF from the Cascades to the Wyoming and northern Colorado Rockies. Topographical forcing combined with PWs near a half inch will also aid in precipitation production. The highest totals appear most likely for northwestern Montana on Friday, and northwest Wyoming on Saturday, with most of this falling as snow. ***ArkLaTex region*** Warm air advection from the Gulf of Mexico increases Saturday night across northern Texas, eastern Oklahoma, and Arkansas as moist return flow from the Gulf of Mexico results in PWs rebounding to near 1.25" over this area. A strong cold front will be crossing the region during this time and will result in enhanced moisture convergence and models generally indicating from 0.5 to 1 inch of rainfall, with a few ensemble members indicating up to 1.5 inches. To account for this possibility, a marginal risk area was introduced from extreme northeast Texas to extreme southeast Missouri, and these areas have received significant rainfall over the past couple of days. However, the best dynamic support aloft should be well to the north across the Midwest and Great Lakes region and this will be a limiting factor in the amount of rainfall observed. Oravec/Hamrick Graphics available at www.wpc.ncep.noaa.gov/qpf/qpf2.shtml