Elevated "convective initiation" in a dry environment

One challenge is nowcasting convective initiation when it occurs within environments which are very dry, and/or when the convection is substantially elevated from surface-based heating, leading to convective clouds which fail to produce echoes of 35 dBZ intensity at ground level. In these instances, a CI nowcasting algorithm that identifies this convective development technically fails as the traditional radar-based CI definition of a 35 dBZ echo (whether at ground level or at the altitude of the -10 C isotherm) is never met. Yet, convective clouds do in fact grow to produce rainfall. This leads to the notion that "you know it when you see it" in terms of CI, but in these situations a hard radar-based definition is less easy to follow.



In this example, the SATCAST (proxy Official AWG CI Algorithm) flagged a cluster of cumulus clouds at 1845 UTC, which are seen in 1 km GOES-13 visible satellite data at 1932 UTC. The first radar echo was seen near 1924 UTC with a maximum dBZ of <20 dBZ. This small cell developed over the Plains, and propagated north-northeatward (to the northeast of Trinidad, CO), and maintained only 10-20 dBZ rainfall. Radar data are shown through 2000 UTC. Also shown is the RUC sounding for near this location which confirm (a) the storm occurred in a dry easterly flow, to the north of a warm from, (b) most unstable CAPE values were near 471 Jkg^-1, and (c) cloud bases were near 680 hPa, with storm tops perhaps reaching ~300 hPa (i.e. it was elevated convection).

This example highlights the sensitivity of the SATCAST algorithm for identifying CI in less than obvious situations (when CAPE values are not high, and when cumulus updraft widths do not necessarily fill a 4 km^2 GOES pixel).

pGLM realtime comparison with MESH and 3D-Var Updraft

Repost from EWP blog...

With warning operations already underway for western Oklahoma, forecasters are deep into their storm analysis.

One of the more interesting features they have been picking up on is the consistent signals between the pGLM lightning trends and values from the MESH (Maximum Expected Size of Hail) algorithm as well as the 3D-Var derived updraft fields.

This was well illustrated by the storm north of Elk City moving from Beckham to Roger Mills county. At approximately the same time ~1900-1915 UTC, the lightning rate increased from 5 to 15 flashes per min (per pGLM grid box, not per storm) as MESH ramped up and updraft increased within the 3D-Var product. Shortly after this increase, both the pGLM and MESH values decreased with this storm (the 3D-Var updraft values also showed this, but with a bit of a time lag).
pGLM flash density and MESH values at 1910 UTC on 19 May 2011
Also of note, prior to losing NLDN data, with the storms seemed to be producing relatively little CG lightning. In this case, the pGLM data was definitely giving a better view of the electrical activity and storm intensity. (1 to 1.75 in hail has already been reported across West and SW Oklahoma).

-K. Kuhlman (pGLM scientist, week 2)

CI along the Sea Breeze southwest of Miami

The SATCAST CI algorithm provided a ~45-51 minute lead time to the first occurrence of 35 dBZ echoes near Homestead, Florida (southwest of Miami). Shown are the 1745 and 1815 UTC SATCAST CI nowcasts on 18 Ma7 2011 (the missing image caused by the GOES full disk scan at 1800 UTC). Composite reflectivity for 1749, 1836 and 1900 UTC verify the new convective rainfall that SATCAST first identified as developing cumulus clouds (see circles on images). Ongoing light rainfall near Miami, and scattered across far southern Florida, between 1749 and 1900 UTC is below 35 dBZ intensity and was not that nowcasted by SATCAST.

PGLM products in AWIPS

Forecasters are now examining the PGLM products within their AWIPS workstations during warning operations with several storms over SW OK. The forecasters have created a 4-panel display that contains the 3 current PGLM products we are providing within the EWP, as well as the traditional radar reflectivity and NLDN detections (see image above). The forecasters currently have access to an instantaneous flash extent density (top right corner), a 60 minute sum (bottom left corner) and a 60 minute max track (bottom right corner). We are also working on providing the forecasters with a flash initiation density product, which will hopefully be available next week.

First CI of the day...

So unlike yesterday, activity has not waited and is currently ongoing and the EWP has began warning operations over the OUN CWA where a tornado watch has been issued at 1803 UTC. Forecasters were monitoring the GOES-R CI products throughout the morning following the issuance of their initial AFD. A line of cumulus formed along a dryline extending through western OK down into TX. At 1702 UTC, UWCI flagged a towering cumulus cloud for "pre-CI growth" (see image below) near the TX/OK border. SATCAST did not flag this storm and at this time radar reflectivity had already exceeded 35 dBZ for this particular storm (see image below). It is likely that the storm initiated between the 15 minute scan period from 1645 UTC and 1702 UTC. UWCI continued to flag this storm during the 1715 UTC scan for "CI ongoing". Also at 1715 UTC, SATCAST flagged a storm just north of the initial storm which continued to form a 35 dBZ echo at 1754 UTC, providing a 39 minute lead time on radar. The southern initial storm had it's first CG flash at 1800 UTC, providing about an hour lead time for the first occurrence of CG for the UWCI.

4-panel CI display (top) and radar reflectivity (bottom) for 1702 and 1707 UTC respectively.
4-panel CI display for 1715 UTC
Unfortunately, as it always seems, we lose satellite data for a 30 minute gap during initiation period, so the EWP forecasters have moved into warning operations using radar data as storms continue to develop northward along the dryline. Hopefully at some point this afternoon/evening we will get a chance to examine the overshooting-top and thermal couplet products in warning operations, as well as the PGLM as the storms move a little further east into the OKLMA domain.

Simulated satellite imagery

The Convective Initiation group compared simulated ABI and current GOES water vapor imagery to examine the dry line over Texas and western Oklahoma and to evaluate the performance of the NSSL-WRF through early morning. The presence of lee waves downstream of the mountains in extreme southwest Texas was noted on the imagery. Overall, the NSSL-WRF accurately depicted most of the large-scale features, with the usual differences in the locations of individual cloud and convective features. The simulated imagery indicates that convective initiation will occur later in the day along the dry line from south-central Kansas to north Texas. The dry line was also evident in the mid-level (band 9) water vapor imagery prior to convective initiation.

-Jason Otkin

Oklahoma CI 18 May 2011

Forecaster comments reposted from EWP blog...

"2330Z Update: UAH CI indicating possible development across Garvin and McClain counties. There does appear to be an HCR in this location but we believe that the edge of thin cirrus clouds are causing algorithm to detect cloud cooling that is not real. Looks like we are going to move operational area to eastern Colorado where there is a lone supercell. kbrown

2230Z Update: Got a hit off of the UAH CI over southern Comanche county where there are weak echoes aloft (15-20dbz). These echoes are associated with a wildfire plume, however.

2140Z: Brief UAH CI detected across north-central Oklahoma in cloud streets (1832Z), but no subsequent echoes were detected.

Although there has been several hours of cu/tcu formation near and east of dryline, CI algorithms have not detected CI across OUN domain. From 20Z to 2115Z this is actually a good thing since no echoes have developed (good case of low false alarm). Before the CIMSS CI became contaminated with Ice Cloud Mask, there were a few hits for CI on leading edge of incoming cirrus across northwest Oklahoma between 19Z to 20Z. No echoes were subsequently detected.

Isolated CI hits from the UAH algorithm did accurately depict some elevated echoes over the OK/AR border but no lightning occurred. Leadtime for echoes was 15 to 30 minutes."

kbrown

A Visit By Don Berchoff

Reposted from EWP blog...

"As we waited agonizingly for convection to initiate in our Central Oklahoma Domain (and as of this writing, we are still without storms), we were paid a visit by Don Berchoff, Director of the NWS OS&T."



"Our discussion with Mr. Berchoff included:

- the need to continue improvements in Convective Initiation (both sensor and model based)
- using multi-sensor blending for more holistic storm interrogation
- his thoughts on the coming of AWIPS2 and the utility for extensibility
- the need for improved analysis and visualization tools in light of the 12 fold increase in data flow

“Fortunately” the weather “cooperated” with us and everyone in the room was able to engage in the discussion."

-K. Manross :: Week 2 EWP Weekly Coordinator

Potentially missed CI target... but with a silver lining

As we have seen over the past couple years during the Spring Experiment, the bane of the satellite community's existence has struck again. It's 2100 UTC and that means we have another 30 minute gap in satellite data right during initiation period... so any updates in the imagery and the CI products will have to wait until after 2115 UTC, and our last image was at 2045 UTC. During that period the cirrus is likely to move over the area, inhibiting any of the CI products from detecting the clouds underneath... We will continue to monitor the area, however it seems like we may have just missed our target.

However, there is a silver lining to this event. Forecasters have been watching the CI products constantly over the past couple hours and have noticed no false alarms over the area of developing cu field for either of the CI products. There were a few false alarms with the cirrus overrunning the surface causing false cooling with the UWCI, as well as a couple false alarms with some of the stratus area in eastern CO with the SATCAST (UAH CI) product. The forecasters mentioned that the fact that these CI products were showing no signals over the cu field of interest provided them with increased confidence that CI was NOT occurring. Increased confidence is a good thing, whether it leads to exciting weather or not.

Monitoring CI over OK...

EWP NWS forecasters have begun real-time operations within the HWT. We are focusing on the UWCI, UAH CI, and Nearcast products over the domain waiting for convection to develop along a dryline extending from central OK down across the Red River. An area of cumulus clouds have begun to form along the dryline and we are waiting for further development. An area of thin cirrus (our worst enemy) is moving in rapidly from NM and will eventually impede upon our ability to detect CI from the satellite based products (see image below). We are hoping that CI will occur prior to the cirrus entering the area. Below is an example of what we are looking at, with the UWCI and ice cloud mask overlaid on visible satellite imagery. A false alarm has occurred due to the thin cirrus being misclassified by the cloud typing algorithm and created a false cooling event as it passed over land.


The EFP is also monitoring CI on the CI desk and have been looking at the Nearcast product extensively over the area. They have noticed that the Nearcast has been showing a band of moisture and increased instability potential aligning itself with the dryline and then being advected quickly east. They are concerned that if this area of moisture and instability moves too far east before CI occurs that the potential for any severe convection will be limited. I explained to them that the RUC winds may not be truly representative of what will really happen and to monitor the new forecast that came in at the top of the hour to see if this trend continues.