Winter 2011-2012 Northwest Flow Snow Event Reviews

This post is a bit off-season, but I want to remind all interested parties that a couple of event reviews from this past winter are available on the Northwest Flow Snow Discussion Group’s web page.

http://www.erh.noaa.gov/gsp/localdat/NWFS_discussion_group/nwfs_discussion_group.html

Or, directly here:

• The Northwest Flow Snow Event of 11 February 2012
• The New Year 2012 Northwest Flow Snow Event

The Northwest Flow Snow Discussion Group is a spinoff from previous NCSU/NWS CSTAR activities.

Larry Lee, WFO GSP

Boundary impacts with TD Beryl

Tropical Depression Beryl is beginning to move northeast out of Georgia today, and is forecast to track across eastern SC toward the NC coast tonight and Wednesday.  A cold front and a line of prefrontal convection are currently approaching the Appalachians from the west, and the remnants of the convection are forecast to move into the Piedmont later tonight.  The true front is forecast to hold back to the north and west.   We have been monitoring each forecast and noting how models are responding to the potential interaction of TD Beryl and the outflow/convergence zone, and NC State student Jordan Dale was actually able to come over to the WFO in Raleigh to look at some of the observations and forecasts with us (a great benefit of the student intern course we are able have here!). 

It doesn’t appear that the boundary interaction with TD Beryl is going to have significant impacts over Central NC, but rainfall totals of 3-5 inches are expected over Coastal NC and we expect some enhancement of rainfall from the boundary overnight into early Wednesday.  Hopefully a short review will follow after the event.

Inland TC Wind Best Practices for NWS Forecasters

The group investigating the inland wind fields associated with tropical cyclones (TCs) has developed guidelines for the upcoming hurricane season that NWS forecasters can use in an effort to help improve TC wind speed/gust forecasts. The initial guidelines are based on a climatological study of past TCs moving through the Southeast/Mid-Atlantic region and some numerical modeling work.  The following are the Powerpoint slides that have been created as an initial R2O (research to operations) product.

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Hurricane Irene Gust Factors

We have been working on a case study for Hurricane Irene and a part of our case study will be examining the wind, wind gusts, and gust factors for the storm. Recall that Irene made landfall near Cape Lookout NC at 8:00 am EDT on 27 August with maximum  sustained winds near 85 mph. Irene tracked north northeast over eastern NC/VA before  re-emerging in the Atlantic near the southern end of the Chesapeake Bay at around 6:00 pm EDT on 27 August.  The track of Irene is shown in the 2011 North Atlantic tracking chart.

Map of the 39 METAR locations in VA, NC, and SC used in the wind and wind gust analysis for Hurricane Irene

NC State student volunteers Rebecca Duell and Lindsey Anderson completed a great deal of the data retrieval and chart creation that will be used in the case study and shown below.  The students created charts containing sustained winds, wind gusts, peak winds, and gust factors for 39 METAR locations in VA, NC, and SC. The locations examined are shown in the figure to the right as stations with a yellow box around the station ID and the individual plots of sustained winds, wind gusts, hourly peak winds, and gusts factor for each of these stations is shown in the table of links further below.

The chart below shows the sustained winds versus gust factors for the 39 stations examined. Note that these values are taken from the standard hourly observation at each location, and not the hourly peak wind gust.  It should be noted that this summary chart includes all of the stations with varied locations, exposure, wind trajectory, etc.  In examining the winds and gusts factors for Irene, several items can be noted.

1) The minimum gust factor observed was generally 1.2 with only 10 out of a possible 1055 observations accompanied by a gust factor of less than 1.2 and just 27 occurrences had gust factors less than 1.25.
2) The gust factor generally decreases and converges with increasing sustained wind speed
3) There is a large variation in gust factors. For lighter sustained winds of 20 MPH or less, gust factors varied greatly from 1.2 to 2.75.
4) At more moderate sustained winds of 20 to 30 MPH, gusts factors ranged from 1.2 to 2.0
5) At stronger winds of 30 MPH or greater, the gust factors were more consistent, ranging from 1.2 to 1.7
6) A one size fits all gust factor did not apply during Irene

Chart of sustained winds versus gust factors for the 39 stations examined during Hurricane Irene

In general, the chart above demonstrates an inverse relationship between the wind speed and gust factor. There are several reasons for this including the reduced frequency of stronger winds, the tendency of the stronger winds to be located near the coast with on-shore exposure or reduced surface roughness, and reduced mixing near the core of the storm.

Individual plots of sustained winds, wind gusts, hourly peak winds, and gusts factor are available via the links below.

KAKQ | KASJ | KCHS | KCPK | KCRE | KECG | KEDE | KEWN | KFAY | KFKN | KGGE | KGSB | KHNZ | KHSE | KILM | KISO | KJNX | KLBT | KLFI | KLHZ | KMEB | KMQI | KMRH | KMYR | KNCA | KNGU | KNKT | KNTU | KOAJ | KONX | KORF | KPGV | KPHF | KPTB | KRDU | KRIC | KRWI | KSFQ | KSUT | KTTA

A histogram of the frequency of gust factors during Irene is shown below.  Note the very few occasion of gust factors of less than 1.2 with a large fraction (53%) of gusts factors occurring in the 1.3 to 1.6 range and 76% of gust factors occurring in the 1.3 to 1.8 range.  The average gust factor among the 1055 observations was 1.60 with a median value of 1.54.

Histogram of the frequency of gust factors observed during Hurricane Irene

Several studies have examined the gust factor while exploring important local influences such as surface roughness and exposure while considering the influence of time averaging periods.  One study from Harper et al. (2008) noted the different gust factors for the peak 3 second gust occurring within a one minute period for various exposures:

In-Land (roughly open terrain) 1.49
Off-Land (offshore winds at a coastline) 1.36
Off-Sea (onshore winds at a coastline)   1.23
At-Sea (offshore > 20km) 1.11

Harper, B. A., J. D. Kepert, and J. D. Ginger, 2008: Wind speed time averaging conversions for tropical cyclone conditions. Proc. 28th Conf.Hurricanes and TropicalMeteorology,Orlando, FL,Amer. Meteor. Soc., 4B.1. [Available online at  http://ams.confex.com/ams/28Hurricanes/techprogram/paper_138064.htm]

TC Boundary/QPF Hurricane Ernesto Case Study Update

Manual surface analyses of pressure and temperature have been completed every six hours from 0000Z 8/31 to 2300Z 9/01 using METAR observations from SC, NC, VA, and (a few) offshore. These analyses were then compared to MSAS 2-D surface frontogenesis and equivalent potential temperature plots to better confirm the frontal location.

Before Ernesto made its second landfall in NC, an MCS developed in NC and VA along the cold front pushing through the SE US on 8/30. Using Galarneau et al.’s (2010) criteria for PREs: 1) Reflectivity ≥ 35 dBZ for at least 6 hours, 2) Rainfall ≥ 100 mm (4”) in 24 hours, 3) Clear separation between PRE and TC rainfall shield, and 4) Deep tropical moisture from TC advected into PRE, it does appear that the MCS on 8/30 met all these criteria. Futhermore, Galarneau et al. (2010) classified the MCS preceding Ernesto as a PRE.

After the MCS dissipated, the cold front stalled along the coast around 0600Z 8/31. As Ernesto approached the coastline, the coastal front strengthened and became entrained into the circulation between 0000Z and 0600Z 9/1. As Ernesto moved inland, it began transitioning into an extratropical cyclone with a split frontal structure developing by 0600Z 9/1:

An occluded front formed by 1800Z 9/1 as the extratropical transition process was nearly complete:

The presence of a frontal boundary provided enhanced forcing for lift, particularly to the east of the TC center where a moist southerly flow was lifted. In the vicinity of frontal lifting nearest the TC center where the circulation and resultant lifting was strongest, a region of enhanced radar reflectivities NE of the TC center led to a swath of extremely heavy rainfall producing storm total accumulations in excess of 10 inches through eastern North Carolina and Virginia.

0600Z 9/01:

1800Z 9/01:

Storm Total Precipitation:

The next steps will include looking at upper air charts for the presence of any jet features. Additionally, model predictability for this event will be examined. Specifically, can the models replicate the frontal boundary structure and precipitation asymmetries? What model physics, resolution, and initial conditions best represent the boundary structure and precipitation distribution?

 

 

 

Classic Severe MCS “Not-crossing” Case this morning

No detailed review here, but thought I’d just take this opportunity since we had a strong MCS (mature bow echo) approaching the west side of the southern Appalachians this morning, with a history of severe weather (see below), to remind folks about some past research associated in part with previous CSTAR efforts. WFO RNK and SPC originally began looking at several years of severe MCS events approaching the Appalachians (50 cases total) to see if we could determine any important environmental clues that would help determine the likelihood of severe weather with these events crossing east of the mountains. A link to those original efforts is here:

http://intranet.wrnk.noaa.gov/reference/MCS/main%20page/severe_mcs_reference%20guide.htm

Following this, and based in part on this database RNK identified, Casey Letkewicz and Matt Parker looked at this problem more thoroughly, including some numerical simulation work. The link to a Wea and Fcstg article on this work is available here:

http://journals.ametsoc.org/doi/abs/10.1175/2010WAF2222379.1

The gist of this is that in most cases, downstream instability is the most important parameter in determining if the MCS is able to continue producing severe weather east of the mountains, but weaker wind shear downstream may also play a role. As such, climatology favors these systems crossing as severe during the afternoon and evening hours, vs. the early morning hours. A couple of exceptions were noted where a severe MCS did not make it across during the day when there was some degree of damming to the east, and another where a severe MCS did make it across overnight when instability remained high due to anomalous moisture pooling along an outflow boundary.

This morning, the MCS/bow echo was classic in appearance with an apparently strong cold pool and trailing stratiform region, and several wind and hail reports across KY/TN, but with sfc-based instability zero with a nocturnal inversion…very few reports were observed even in the higher elevations, and really none at any appreciable distance east of the mountains (a couple in the Asheville area).

 

 

If one simply viewed the loop of the reflectivity (not shown here), despite a weakening to the convective leading edge and apparent rear-to-front flow withing the MCS, one could argue that the system itself effectively crossed the mountains, yet clearly severe weather did not cross, and that was the focus of the above research efforts.

Mat Parker may have some additional thoughts on the findings of the above studie, and previous work he has done on MCSs in this region.

Update on the Observations of mergers between Squall Lines and Isolated Supercell Thunderstorms

This is an update to a previous blog post – Convective Storm Evolution Following Mergers between Squall Lines and Isolated Supercell Thunderstorms

Last fall, Dr. Adam French, provided WFO Raleigh with a presentation on his research that examined mergers between squall lines and isolated supercell thunderstorms. A summary of the presentation along with links to a recording of the presentation along with the power point and a PDF of the presentation are available in the original blog post.

The recently released April edition of Weather and Forecasting includes an article authored by Dr.  Adam French and Dr. Matthew Parker that formally shares this research -
Observations of Mergers Between Squall Lines and Isolated Supercell Thunderstorms

The paper describes the evolving severe weather threat during the course of the squall line and supercell merger, with the details of the evolution and severe weather threat largely dependent upon the background environment. The paper provides several worthwhile conceptual models and some excellent context for radar operators to be aware of during warning operations.