create table test
(
obsdate datetime,
obsvalue  int
)

insert into test(obsdate, obsvalue)
select '2010-01-01', 100
union
select '2010-01-02', 101
union
select '2010-01-03', null
union
select '2010-01-04', null
union
select '2010-01-05', 104
union
select '2010-01-06', 105
union
select '2010-01-07', 106
union
select '2010-01-08', null
union
select '2010-01-09', null
union
select '2010-01-10', 109


declare @from datetime
declare @to datetime
declare @minDate datetime
declare @latestKnownObsValue int
--set @from = '2010-01-03'
--set @to = '2010-01-05'
--set @from = '2010-01-01' 
--set @to = '2010-01-06' 
set @from = '2010-01-01' 
set @to = '2010-01-10' 

select * from test

--for this result, find the previous non-null value replace with the null value
select obsdate, obsvalue 
from test
where obsdate between @from and @to

select @minDate=min(obsdate) 
from test 
where obsvalue is null
and obsdate between @from and @to 

select @latestKnownObsValue=max(obsvalue)
from test
where obsdate < @minDate

select obsdate, isnull(obsvalue ,@latestKnownObsValue) as obsvalue
from test
where obsdate between @from and @to

SELECT
	t.[obsdate]
	, ISNULL(t.[obsvalue], pVal.[obsvalue]) AS [obsvalue]
FROM
	test AS t

	OUTER APPLY (
		SELECT TOP 1
			[obsvalue] AS [obsvalue]
		FROM
			test AS t2
		WHERE
			t2.[obsvalue] IS NOT NULL
			AND t2.[obsDate] < t.[obsDate]
		ORDER BY
			t2.[obsDate] DESC
		)
		AS pVal
WHERE
	obsdate BETWEEN @from AND @to