Significant Warm Season Precipitation

Events in the Burlington, VT Region

ByEyad Atallah, John Gyakum, and Margaret

Kimball (McGill)Paul Sisson (NWS-BTV)

Motivation The goal of this study was to identify large-

scale circulation anomalies associated with significant warm season precipitation events.

Events were identified using the Unified Precipitation Dataset and then composites

were used to compare the events to climatology.

Data areaFor every date, the maximum over the grid area was found.

This maximum was then used as the precipitation value for that day.

Unified Precipitation Dataset

Availability NOAA CDC online

Time period 1948-1998

Spatial coverage United States gridded .25x.25 degrees

Temporal coverage Daily

stations About 13,000 after 1992 and 8,000 before that. Includes COOP observers!

StatisticsMean 0.45168 in

Standard deviation

0.49831 in

Extreme >2 sigma >1.448305

Heavy [+1sigma, +2sigma] [0.94995, 1.448305]

Moderate [+0.5sigma, +1sigma] [0.70084, 0.949995]

Case Selection

Raw Restricted Percentage of total rain events

Extreme 225 53 1.3%Heavy 462 94 3.5%Moderate 429 61 5.0%

•7 day separation for event independence

•Restricted to post-1963; used NCEP reanalysis

•Heavy and Moderate were limited to 53 cases via random elimination

Data and Methodology• Composites and climatologies were developed

using the National Centers for Environmental Prediction/National Centers for Atmospheric Research global Reanalysis for the period 1963-1998 inclusive.

• Since all events are centered at 00 UTC, diurnal climatologies were developed, and weighted based on the distribution of events in each composite.

Conclusions•Extreme events present a coherent structure dominated by a warm anomaly up to 60 hrs before the event. Troughs in extreme events are more slowly propagating with a positively tilted structure at 00 h.

•Heavy events have a clear synoptic-scale trough/ridge couplet which develops a bit later, representing stronger, but shorter duration forcing for ascent than extreme cases.

•Moderate events have little structure- possibly a result of smearing.

•Precipitable Water shows a greater intensity and geographic extent in the extreme cases then either heavy or moderate. Largest differences in the vertical moisture profile are located between 700-500 hPa.