Here is the NWS's very long discussion of exactly how complicated this storm system is:
.SHORT TERM /Tonight through Saturday Night/... Issued at 408 AM EDT Fri Mar 23 2018 The primary focus will be the potentially significant late season winter storm poised to impact central Indiana beginning late tonight and continuing into Saturday evening. There are still details to be ironed out as we move into the 24 to 36 hour timeframe and multiple items to consider that could drastically change precip type and amounts in any one location. Lot to look at and consider. The first order of business to address is the headline situation. After discussion with neighbors and with confidence levels not yet quite where would like to see them...have chosen to continue with the Winter Storm Watch in its present state with an addition of counties to the southwest based on a southward shift to the potential for heavier snowfall off the 00Z model suite. With impacts in the late second period and...more likely in the third period... think there is still time to fine tune details and determine where headline adjustments to warning and/or advisory would best fit. In addition...the 12Z model suite should be able to capture the system with the full RAOB network as it has comes ashore on the west coast over the last several hours. That should hopefully iron out some of the differences that still exist between various model solutions which we will discuss below. The upper wave will lift up and over the ridge centered over the southern Plains later today and tonight before winging into the region Saturday and gradually becoming absorbed by the deep upper trough centered over the Canadian Maritimes and New England states by late weekend. A surface low will accompany the upper wave...ejecting out of the Rockies this evening then moving E/Se into the lower Ohio Valley by late day Saturday. Overall model agreement is decent regarding the main features with the exception of the NAM which is a noticeable outlier to the north. Overall ensemble guidance is tightly clustered and aligned with the op GFS and ECMWF with the GGEM tracking slightly further south. Rain will be overspreading the lower Wabash Valley during the evening...with precipitation slowly expanding east across central Indiana into the overnight as the initial surge of moisture and lift contends with a notable layer of dry air through the lowest 10kft courtesy of increasing easterly flow. As surface temps fall and evaporative cooling commences in response to the saturation of the boundary layer...will see precipitation transition to sleet and more likely snow along the northern flank of the precip shield after 06Z...with the transition zone aligning somewhere near or just north of a Terre Haute-Bloomington-Seymour line. Northeast of this line...snow should become the predominant precip type through daybreak with the potential for a light accumulation. This transition line is likely to shift northeast slightly during the day Saturday. Within the mix area...freezing rain is certainly a possibility...but expect sleet will be the more prevalent precip type outside of snow or rain based on model soundings. The primary show however really looks to focus during the day Saturday as the upper wave approaches with more substantial deformation and frontogenesis develops and moves across the region. The potential for a period of or bursts of heavy snow will come during the day Saturday and should align in a NW to SE axis along where the most intense deformation occurs just to the left of the 700mb low track. Potential is there for a burst of more widespread snowfall during the morning then after a brief break...more locally heavy snow during the afternoon with the possibility of a convective component. Cross sections continue to show a few hours from late morning into the afternoon where strong vertical velocities and moisture interact with the dendritic growth zone in the presence of conditional instability. With the deformation axis likely to be nearby at the same time...this is the time period where the heaviest snow rates are likely to occur and a potential for a few rumbles of thunder within the heaviest snow bands will exist. Further to the southwest where rain remains the primary precip type...an isolated thunder mention is warranted. Also...an impressive wind field present through the boundary layer wrapping around the system will lead to higher wind gusts Saturday that may peak at 30-40mph. This will make the day even more miserable and in areas where a snow or mix is ongoing...create greater impacts. While all of the above presents a growing confidence in a heavy...wet and impactful snow...low level thermals will offer a challenge as they often do with late season winter storms. Surface temps are likely to rise into the mid 30s over the northeast half of the area with a warmer layer aloft that should warm advection be slightly stronger than currently progged...could cause precip type changes and/or limit heavier snowfall rates. The heaviest snow falling during daylight hours highlights the complexity of the low level thermal profiles even further. That being said based on the forcing aloft and factors mentioned above...feel that rates within the heavier bands will overcome marginal low level thermals and cause problems at times. Precip will diminish as the system moves away Saturday evening with skies clearing overnight. So to summarize...the potential for significant winter weather primarily in the form of snow remains high but some details still need to be worked out. Would like to see an overall better model consensus on the axis of heaviest snowfall...as the NAM remains north and the ECMWF and GGEM have shifted much further southwest on 00Z runs. The overall consensus without the NAM does support a slight shift to the southwest and have followed suit. As it stands at this point...think the axis of heaviest snowfall is likely to set up either side of I-74 by 20 miles or so. The axis will in reality probably end up being tighter but would like to see better consistency before committing to a narrower axis. Will continue with snowfall amounts of 3-7 inches within the heaviest bands with a potnetial for localized higher amounts which are common in banded snow scenarios. One thing to mention...warmer ground temps will likely lead to compaction of snowfall relatively fast...especially once rates diminish. Actual snow depths will likely be lower than snowfall amounts for this reason. This will be cement block snowfall folks. Temps...largely prefer the cooler end of guidance through the period...especially on Saturday as temperatures are likely to be kept down by precipitation and especially in areas where snow or some form of wintry mix is prevalent. && .LONG TERM /Sunday through Thursday Night/... Issued at 353 AM EDT Fri Mar 23 2018 Ridging over the Great Lakes Region will result in dry conditions early in the extended period. However, rain showers will slowly start entering the southern portions of central Indiana early Monday morning ahead of the next low pressure system over the Central Plains. Significant Gulf moisture will be pulled into central Indiana ahead of this system, resulting in heavy rainfall at times on Monday and Tuesday along with embedded thunderstorms in the warm sector. Latest blended initialization highlights these periods well, so no adjustments were needed. The forecast will remain active through the end of the extended period as a trailing cold front moves through on Tuesday night followed by an upper low late in the period. Meanwhile, temperatures will initially start out below normal, but they will quickly climb above normal due to strong warm advection with aforementioned system.
No comments:
Post a Comment